Not sure of the 377/4 layout, but the 377/5 sound very similar: the end coaches are 2+2 for 2/3 of the length + first class (also 2+2) between the driver's cab and the first set of doors. At least one of the the 3+2 centre coaches has a section at the end with 2+2 (no arm rests, though).
Quick question do 377/5s have exactly the same format of seating as 377/4s ie 2+2 front and rear coach and high density 3+2 centre coaches?
Biggest problem with the 377/5s is less seats than a 319 (other than the ex Brighton Express ones), which reduces the chance of a seat when I travel in the mornings.