Possible or good idea? I can guarantee it's possible; propane and natural gases are extremely popular in 2nd World Countries because it's a fraction of the cost of gas and often much easier/more reliable to obtain, and they got loads of these 4Runners like us. I believe it was Moldova that actually has methane fuel stations; Russians and Ukrainians LPG swap literally everything they can
The problem with that is finding information is probably gonna be extremely scarce as a result (nor would I do what they do, as many overseas throw safety out the window). There are a couple American users here that DF gasoline+ethanol that might be of assistance, however I can't remember who they were and haven't seen them around in some time. If you had to start somewhere I'd talk to them and the Jeep guys as mentioned above. It won't get you terribly far as the 4.0 and 3.4 are as different as the vehicles they came out of, but it would be a start nonetheless
Tbh I can't see why you'd want to run both really; gasoline does everything propane can but better. Propane has a leg up when it comes to octane but our engines are such low compression you'll never see the benefit of it. Even the guys who did ethanol+gas builds I'm pretty sure really only did it because they were supercharged and could benefit from lower combustion temps. If you are supercharged I would greatly reconsider with how much hotter propane burns
Ultimately it sounds like a ton of work for not a lot of product