In many references concerning Soviet long-range aircraft available during WWII, you will find notice of a "show aircraft" Stalin sent on various long range trips for the purpose of propaganda.
Up until a decade or so ago the "official" position taken by Janes and others was that the particular aircraft never existed and was certainly never observed in the United States.
It landed at Bolling AFB in Washington DC in 1944 and was seen by thousands of people. All the "official" position folks finally agreed it really did exist so it's OK to talk about it. At the same time no one knows how many of that model the Soviets actually had.
The only four-engined bomber the USSR had in WWII that was produced in any numbers (about 150) was the Pe-8. The "mysterious" aircraft you mentioned above WAS a Soviet PE-8 which was used to transport the Soviet Foreign Minister Molotov to both Britain and the United States in 1942. There were no other super-secret Russian four-engined bombers in WWII, they never existed. Once the decision to duplicate the B-29 was made, all other post-war strategic bomber programs were halted.
"Obviously they didn't engage in strategic bombing (since the US wasn't sending them any bombs among other reasons), but they didn't need to. The US and UK did the job for them."
"That doesn't mean they weren't capable of doing it."
They were not capable of conducting strategic bombing, their resources were tied up in the Red Army and its supporting tactical air force. They had neither the strategic bombers, nor the long-ranged fighter aircraft necessary to conduct such operations. While the Soviets were fairly well put for basic oil stocks and refining, they did not have the ability to refine sufficient quantities of high-octane AVGAS required for high-altitude operations without being supplied by US Army lend-lease POL logistics operating in Iran during WWII.
"Now, back to the issue of how many atom bombs the US had ~ there was no major production line set up in 1945 to produce large quantities of atom bombs. That said, we note that the Russians didn't have a major production line set up until 1947 to produce large quantities of the TU-4."
Of course there WAS a production line set up to generate plutonium cores for the "Fat Man" model of the US nuclear stockpile! Did you think that the US had invested 2 billion (1943) dollars just to make five atomic bombs in 1945? The "Little Boy" uranium gun-type atomic weapon first dropped on Hiroshima was a one-off model, never produced again. All of the other US atomic weapons were of the plutonium-implosion "Fat Man" model. So the first bomb was tested in the US during July 1945. Two more atomic weapons were dropped on Japan in August 1945. One more atomic bomb was being readied for Tokyo for late August 1945; it was never delivered. At the end of calendar year 1945 the US had two "Fat Man" type nuclear weapons in its inventory out of the five produced in 1945, however if Japan had not surrendered the nuclear 'production line' that you insist did not exist was designed to produce 7 plutonium cored nuclear weapons per month.
"A third bomb was being shipped from New Mexico, target Tokyo, when the war ended. Production was geared to seven per month with an expectation that 50 bombs would be required to assure that an invasion would not be required. Release of radiation from the untested Hiroshima bomb, designed as the original gun-type and made of uranium, was a surprise. The radiation range was expected to be within the blast radius, that is, a lethal dose of radiation would only kill those already dead from concussion. The Alamogordo bomb test and later production were of the more complicated plutonium, yet cleaner, implosion device."
Source: WW2 Pacific: Little Known Facts: Atomic Bomb -- Allies
dvwjr
We didn't know we had a real need to build more of them until the Russians set one off!
Notice that to build one nuke it costs an immense amount of money. To build two of them it costs quite a bit of money, but not much more than just to build one. And so forth. On the other hand, to build dozens, hundreds, and then thousands of them you have to mine the heck out of sources of uranium.
You might try to find out how many tons of processed yellow-cake we had on hand at the end of August 1945.