I have had a couple of assaults and look after a few others.
I wouldn't bother with a 4m as the actual hull is only about 3m long, try as we might we couldn't get one to run nicely with a 30 Honda on it.
The 4.5 and 5 are ok, I ran a 5m with a 90 yam on it which was fun (43kn on gps)
I've also run 5m assaults with a sixty two stroke yam, a 70 evinrude and 70 yam, also with 40 and 50 Hondas.
The assault doesn't have a particularly shallow v hull BUT the tubes are mounted very low and tend to drag in the water and so give a lumpy ride in a chop. However as a safety boat its easier to drag people into than many other ribs because of the low tubes.
There is another humber model with a much shallower V, not sure of the name but I saw a new 5m one last year.
The destroyer 5m is a much better sea boat though considerably heavier.
We bought a new 5m assault last month direct from humber, without a console for £3600 inc vat. With a two man sports console it would have been £4500. So humbers are very keenly priced and tough as old boots too.