Originally Posted by
HSLD
I've only flown the 737-200 so I'm no expert, but the -400 is he same generation as the -300 and -500. I'm sure there are differences as you've pointed out, but it's the same type.
I think the -400 has a different environmental system as well. Look up this site they seem to cover everything the guppy has to offer from the -200 to the -900ER
The Boeing 737 Technical Site