Here's my calculation:
Assume 5th year 's rate is 6%. Your house is 400K and 20% downpay.
Option 1: 4 years 3.8%, 5th year 6%.
Option 2: 5 years 4.2%.
End of 5 years:
[FONT=宋体]Total 5 Years payment[/FONT]
[FONT=宋体]$103,445.00[/FONT][FONT=宋体]$103,476.93[/FONT]
[FONT=宋体]Total 5 Years Principle Payment[/FONT]
[FONT=宋体]$40,189.00[/FONT][FONT=宋体]$40,289.12[/FONT]
[FONT=宋体]Total 5 Years Interest Payment[/FONT]
[FONT=宋体]$63,256.01[/FONT][FONT=宋体]$63,187.80[/FONT]
[FONT=宋体]5 Year Balance[/FONT]
[FONT=宋体]$279,811.00[/FONT][FONT=宋体]$279,710.88[/FONT]
From my opinion, not much of a difference if 5th's rate is 6%. But I agree with windmill, take the lowest rate for now and change to variable if prime is higher at the end of the term.
Hope this helps you.