My experience of Mk4s is slight and was mostly in the 1990s when they were first introduced on the ECML.
The main feature I remember was the loud metallic squeal when you tried to use the seat-back tray tables.
I used an ex-GC set today (HD06) on 1230 Manchester-Crewe (1V42), and was much amused to find that despite two expensive upgrades, the trays still squeal, and just as loudly. You could tell when anybody in the half-carriage moved their trays.
But at least the ride was comfortable, and 4 carriages is much better than the 2-car set offered on the 1130, or the 0-cars on the cancelled 1330*.
* seemingly the incoming service was cancelled at Hereford.