I own both a L412 and L450, I assume both are 4 cabin
versions. 4 couples will fit on the 420 very nicely, in fact the forward cabins on the L420 are in my opinion more roomy than on the L450 although the beds are a bit higher and mounted East West. The L450 wins hands down with the aft cockpit
space, also sunbeds in front of the fly bridge. More lounging space on the forward deck
area on the L450 as well.
If your cruising area offers a lot of opportunity for off boat eating and drinking then I would go L420 and each couple has an extra 1000 euro to spend. If you will be predominantly on-board for the whole charter
go the extra entertainment room of the L450. Either way you will have a lot of fun.