[Hermann the Cherusker]

I guess my question to you, and other very devout Catholics, is what constitutes a grave reason not to have more? I know four to a bedroom isn't a reason. Is the inability to pay for a Catholic school a valid reason? Are stressful pregnancies with much medical intervention a valid reason? What if you are not a very good mother/father and cannot handle more?

Anything you list above, including 4 to a bedroom, could constitute a just reason. A married couple should "thoughtfully take into account both their own welfare and that of their children, those already born and those which may be foreseen. For this accounting they will reckon with both the material and the spiritual conditions of the times as well as of their state in life. Finally, they will consult the interests of the family group, of temporal society, and of the Church itself" (Gaudium et Spes, 50).

Or is it up to the individual conscience?

Yes, if the conscience is well formed in its moral obligations. To those with doubts, they should consult a Priest.