Where does too much overflow production go?

[mnky]

Example:

Unit costs 36 Production, but i do 40 per turn. The production is repeated with the same units. Where does the remainder go? Is it stored up forever until i use it or does it never go past 4 overflow production?

It would be nice if its stored up. It would actually be bad for Space Marines, because they are very likely to produce this way. Other Races dont do this nearly as often because of multiple cities.