Let's say the information you provided relates to a 3/1 ARM with a 30 year term, I would say you would disclose the following:
36 payments based on 6.00% (premium rate)
12 payments based on 4.00% (2% periodic cap)
312 payments based on 3.75% your fully indexed rate.
Your software would then compute a composite APR taking these rates, payment streams, and the finance charges into consideration.
Opinions expressed are my own and not necessarily those of my employer. They are not legal advice.