A 171 tonne vessal with only four missiles? Something does not seem right.
I believe the reasoning is that in a real (hot) war you would possibly only have the opportunity to use a certain number of missiles before something (maybe an opponent's surface craft, submarine, air strike) took you out. Thus, in terms of offensive anti-ship capability, most surface combatants did not necessarily carry a huge warload of ASMs.
An even better case in point is the ex-Soviet Sovremenny class destroyers, with were big ships of over 7,000 tons (compared to the 171 tons for the Taiwanese gunboats) that, in terms of offensive anti-ship weaponry, carried 'only' 8 missiles. Now, these missiles were the supersonic Sunburns, but they were only 8. Reason? Soviet doctrine was that no surface ship would really have survived a major confrontation with NATO. The same, by the way, applied to NATO surface ships ....an interesting read would be the projected survivability of surface ships, in particular aircraft carriers, in a WW3 conflagration if the Cold War had heated up to a boil. Another interesting read (especially for those FReepers who believe all the US needs in terms of air capability is the A-10 Thunderbolt) is the survivability projections for A-10s should the Soviets have invaded through the Fulda gap (a popular joke between A-10 pilots and AH-64 Apache pilots was on who would die first). Had the Cold War heated up, that would have been one war with no real 'winners' ...just a question of who is more irradiated than the other.
Anyways, going back to the Sovremenny destroyer, it is a big ship that only packs 8 ASMs as offensive sea-surface weaponry. Why? Because it will only have one real chance to use them before a LA class submarine would have got it. Thus, it only needed 8 missiles, and those missiles had to be really capable (hence the development of the SS-N-22 Sunburn). After it launched the missiles, preferably at an American ship (e.g. a carrier), the Soveremenny was then disposable. Sure, it had anti-aircraft missiles, torpedo decoys, and other defenses to try and survive back to base, but in a real fight its survivability (like ANY surface ship) was really next to zero (and chances are its base would have already been nuked), thus its only purpose was to survive long enough to launch those 8 missiles. Everything was to make it last long enough to launch those Sunburns at an American/NATO ship, and preferably a 'big' ship that would justify the loss of the Sovremenny.
Same thing with the Taiwanese. They are facing a far more superior enemy, and the survivability of their surface ships is also next to nil once a major war starts. In the same way, 'going back to base' to replenish missiles will also be more or less moot since those bases will have been targeted by Intermediate Range Ballistic Missiles (most probably with conventional bomblet warheads) and taken out in the opening sequences of a real China-Taiwan spat. However, if that gunboat (especially with its new stealthy applications ...even if they do not work as advertised every little thing helps) can have an opportunity to launch those 4 missiles (and chances are they will be launching them at more expensive Chinese ships), then it has done its job even if it is sunk. Assuming each gunboat knocks out (just) one more expensive ship before it is sunk, that is a rate of attrition that is in Taiwan's favor ...as long as they have enough gunboats and the Chinese do not want to lose too many ships. That is the logic they are using, and the reason why the gunboat only has 4 missiles. Taiwan does not have the money to spend matching ship against ship with China, and rather than have one really capable Arleigh Burke type AEGIS ship to match up against China's AEGIS-esque phased-radar guided-missile destroyers, the Taiwanese are opting for cheap gunboats (many of them) that have their survivability (somewhat) enhanced with stealthy coatings, and packing advanced anti-ship missiles that gives them (something of) a chance against more capable Chinese ships. While China still has an (overwhelming) advantage, the Taiwanese stratagem still gives them enough of a headache to not start something just for fits and giggles. Something akin to a bear keeping away from a wasp ...yes, it could crush the wasp easily, but it would rather not get stung.
By the way, this is the reason why the Chinese do not want the Taiwanese, under any circumstances, to get their hands of modern advanced diesel-electric submarines. Think of it this way ...if a number of gunboats armed with 4 missiles can cause a major headache for an attacking force (whether it be China's phased-array destroyers, or incoming troop-transports/amphibious vehicles), imagine what just a dozen advanced D-E submarines with AIP technology and modern ASMs like the Klub supersonic missiles could do? Imagine if Taiwan had 12 Kilos, or 12 U-214s, or 12 Scorpenes? Those 12 submarines, from any country be it France or Russia or Germany, in Taiwan's hands would effectively and completely make the chance of any attack by China on Taiwan almost impossible. The survivability of a modern D-E AIP submarine in the Taiwanese Strait would be exceedingly high (especially compared to surface vessels), and with modern sub-launched anti-ship missiles like the Klub, and modern programmable anti-ship mines, the Taiwanese would be able to mount an effective defense WITHOUT the need of American (or other foreign) assistance. Would China still have an edge? Yes, they would still have an immense edge, but instead of a bear against a single bee it would be like a bear against an entire bee hive. Can the bear still win? Yes, it can ...however the cost implications just went up significantly.
This is why any nation that has Taiwan has tried to get submarines from (such as the aforementioned 3 - Germany, France and Russia, who make the best D-E submarines in the world) get pressure from China not to. Japan also makes some very good D-E submarines, but they do not sell to anyone (I believe it may be in their constitution, but Japan simply doesn't sell technology ...the only country they have shared some technology with is the US). South Korea also makes a version of the U-209, but I don't know why they have not sold to Taiwan (maybe Chinese pressure not to?). Then there is the US ....which has not built any D-E submarines in decades, and I doubt they are about to sell nuclear submarines to Taiwan anytime in the next millenium! Thus Taiwan is shut out.
However, the day they get just a dozen modern D-E submarines is the day any threat of invasion goes down to nil.