Also I can't really speak about 737-800 & 900's, however what I can tell you is that you need 1 cabin crew member per 50 passengers.
I believe Ryanair 737-800's have 189 passengers, hence 4 cabin crew.
From doing a bit of searching it looks like the 737-900 also has 189 seats but it has a much longer range, hence the benefit for some operators.
SW