I have a DK40se and both the standard 72" bucket and a WRLong 60" 4n1. Unless you are moving large amounts of mulch or sand I don't think the actual volume capacity of the bucket makes a whole lot of difference. I rarely have a truly full bucket of material in either bucket so losing 25% or so with the 4n1 isn't a big deal.
The WRLong 4n1 is very well built and I do find a few uses for it but frankly it is a waste of money if you already have a standard bucket and grapple. And, if you don't have a grapple before considering a 4n1 you need to seriously consider what you will be doing with the 4n1 that you could not do much more efficiently with the grapple for much less money.
From my experience, the 4n1 is a Swiss Army knife. Nice to have if you can only have one tool but virtually never the tool of choice. The only time I find my 4n1 truly useful is when I have my backhoe mounted and I am digging either stumps or trenches and need to backfill. The 4n1 makes a TERRIBLE grapple substitute for carrying brush because of the top hinge which means that everything below the object that clamps first between the two jaws will simply fall out of the bucket. It is perfectly fine for picking up precisely one branch but that top branch will prevent the bucket from clamping on anything else and as gravity is your enemy with a 4n1 everything else below that branch will not be secured. A grapple has gravity as a friend as the lower tines hold the load and the upper clamping arm just keeps it from falling forward. I've used standard bucket, standard bucket plus toothbar, standard bucket with rachet rake, grapple and 4n1. The 4n1 is more useful than the standard bucket alone but less productive than any of the others for specific digging, dragging or carrying tasks.
The 4n1 can "bulldoze" but that is a really bad thing to do with a tractor FEL unless you are going very slowly. Much safer to do that sort of work with a box blade. I cannot even remember right now what the fourth function is in a 4n1 but I haven't found it useful (I think they count the ability of the extended front blade to cut when pulled backwards....dangerous to the bucket in my opinion).
Bottom line: if you like Swiss Army knives rather than real tools you will be happy with a 4n1. Otherwise get the real tools first. The exception to that statement is for someone who is trailering a tractor and cannot take multiple implements with him to do a job. 4n1s make perfect sense for a general contractor for example.