Depends. What are you really trying to achieve? What will it be used for and what kind of budget are you looking at?
In reality you don't need to 'lift' to get more flex. In fact you already have good ground clearance (same as a Defender if on same tyres). Are you willing to trim wheel arches?
I don't know of +4" shocks.....
BTW - I suspect at the front you won't get enough flex from the radius arms to even max out +2" shocks, so +4" will not give you anything any extra.
Also note that more than +2" shocks will likely need revised upper shock mounts, else you'll run the risk of bottoming out the shocks. Which will reduce over travel and cause damage. Extended bump stops might sort of solve this, but again only serve to actually limit articulation and flex, not increase it!
Long springs will mean you may not need dislocation cones (which is a compromise IMO). Soft springs also tend to flex better, but depends how heavy your rig is, what you haul and if you tow. Red/White Range Rover springs all round work quite well for reduce loading (softer rear).
Flatdog also offer some good springs, although I'd possible mix and match between the +2" Discovery, RRC and Defender ones to get the right spring rates for your needs. Disco's normally run dual rate rears, but no reason you couldn't run a Defender or RRC spring instead.
https://www.flatdoguk.com/disco-1-+2/hd1~454
https://www.flatdoguk.com/rr-classic-+2/std~459
Lot's of +2" shocks to choose from, although the Britpart celluar dynamics are pretty good for not a huge sum of money. Have also got on well with Procomps.
In terms of more flex than this.... The rear is quite easy to get more flex from, the A-frame works really well and longer shocks will give more travel. But as said, you'll need revise the upper mounts to allow the shock to safely compress. More flex than the +2" also means you'll want to look at a new wide angle prop.
The front is much harder to get more flex from, which is a bit more of a trade secret I'm afraid
Depending on your use, you might consider an Ashcroft rear ATB diff. Similar money to a long travel suspension setup and highly effective off road.
