|How about this:|
=IF(A1="FT",IF(B1>=4,15,IF(B1>=2,10,IF(B1>=1,5,0))),"Not Full Time")
Seems to me that you only need to check for FT once and if it's TRUE, then start checking for your Years Employed. If A1 = "FT" is TRUE, then the formula will stop at the first B1 ("Years Employed") that returns TRUE.
If A1<>"FT" then the formula will jump to the very last value_if_false.
I used "Not Full Time" as the value_if_false for the IF(A1="FT") test just so that you could tell which argument was which. You can replace that with a 0 if you'd like.