I doubt that TW4 will be as devastating as TW3. 3 blindsided the GDI, not to mention the issues Boyle presented. Here, we know that Nod is not beaten, and that the Visitors may very well return.We burned through somewhere in the ballpark of 300 cap goods just in the first few years of the game before getting back to breaking even. And that was AFTER the war was over and we could rebuild in relative peace, if you factor in the stockpile burndown during the years of war offscreen before the quest actually started we could have easily burnt through 1000 stockpiled cap goods if not more. Our current stockpiles are definitely nowhere near enough to cover another TW3-level event, and even with the thousand-odd stockpiled cap goods we had in the 2040's we were STILL desperate enough to take hardware from the 1990's out of mothballs lest society collapse entirely.
A reasonable lowest bound on the cap goods stockpile is probably somewhere in the 800-1000 range, and really should be higher so that we're not cutting it so close that we need to use 90's era harvesters and refineries next time we get invaded by aliens.
Forewarned is very much forearmed in these situations. We have an entire branch of the military that is almost solely focused on ensuring that the Visitors will receive a much less pleasant welcome should they return. Their confidence is already at decent, and it sounds like @Simon_Jester intends to offer them yet more support in 2064.
Anyways, if we need to get that many Cap Goods in the stockpile, then we are completely screwed if the Visitors show up again in anything except the extremely long term. A loss of 10 Cap Goods won't make that much of a difference over that massive of a timeframe. If you are worried about the possibility of a second alien invasion, I think building up our Space Force is a much better bet than trying to stockpile that many Cap Goods.