Hello, a simple question: cost wise, what would be the cheapest: -traditional poured foundatin and 4" of ridgid foam installed or -icf foundation? thanks

Comparing apples to apples, which your comparison isn't quite, but close, you will be literally within pennies per sq.ft. if you are truly comparing materials AND labor, i.e. paying someone to apply your 4" of EPS. Things to consider, how are you fastening the EPS? Are you truly going to get as good a seal as ICF will provide? How are you planning to finish the EPS?

Go with the concrete and add the 4", I do both ICF and traditional Below ground... traditional is cheaper, no question and can be as good or better than ICF, all you need to do is run the numbers.

You would still be out at least 1" of foam when comparing most ICF's and as Chris noted you would never get the tight seal that you get with poured in place. Any convection movement between the concrete and after applied eps will degradate the r value of the assembly.