Both shocks are capable but the Rc4 is just a more tunable and a racer's shock by that I mean racers want all those little edges, its got those updates from the van and those extra adjustments that make it that much (whether a lot or a little) better.
I have also heard a bit about vans blowing up more than rc4 so reliability might be a issue? not sure. I didn't even ride my van on my bike I switched it out for a rc4 at purchase and damn is the rc4 nice. I like having as much control over the shock as possible it just helps with set up and and getting the shock feeling perfect.
Checking out the website the Rc4 has HSC, bottom out control and bottom out force adjust
HSC is obviously a massive benefit on a bike, When we ride LSC control things like body movement, dives and slow compression, most things like rock gardens and most things we hit on a dh trail tend to activate the HSC in the shock so it is a very valuable tuning adjustment.
BOC - basically this just adjust the progressiveness of the suspension towards the end, this is obviously very helpful, if you are happy with how the bike feels through the travel but need a little more at the end you can change this via BOC.
BOF - this is basically the air pressure in the chamber and does effect the whole shocks feeling, it is best used or thought as a mid stroke support and ending stroke support.
Take the following into consideration.
You have a VAN with LSC rebound and spring rate. If you find you are bottoming out the shock you need to up the spring rate, this might mean your beginning stroke is harsh and you get little sag, HSC on this shock is controlled internally by shim stacks so you can alter HSC but it requires some internal shock configuration . basically you cannot isolate any adjustments.
You have a RC4 that has the right spring rate and you have adjusted your LSC and rebound
Now you find the shock feels like it it blowing through its travel on hits so you adjust your HSC, this solves the problem.
Now you are riding and you find that the bike is blowing through its travel too much again so you decide to up your HSC so it uses less travel, this then makes the bike feel harsh on square edge hits. Instead you turn the HSC back down to how you had it as this is where you want it and instead you add some pressure to the chamber to give more support.
You now ride the bike and this time the shock is using less travel again but the bike also does not feel harsh on square edge or immediate hits. This is the advantage of pressure adjust
Now you further find now big drops or harsh landings are causing the bike to bottom out too often but the bike feels perfect else where so changing HSC or pressure will make the end feel better but make the rest of the suspension feel poor, instead of changing to more HSC or more pressure you can add some BOC to make the ending stroke more progressive. This means you now can leave your HSC and pressure in its perfect position and independently adjust that every end of the travel.
You now have a bike that uses its travel brilliantly but does it in a controlled smooth manner.
As you can see if you don't have some adjustments you need to try and control them via other methods which not only mess with how the bike feels in other areas but they don't really focus on the specific thing you are trying to change.
An Rc4 lets you isolate the forces and sections of the shock so you can adjust them correctly without affecting the other areas beyond what is necessary, therefore you can really tweak the bike and get it performing how you want.