is it possible to swap in a j35a8 into a 4dr?
Possible? Yes. Cost effective? No. You could probably use any standard v6 mounts for a cg but you’ll need a new ecu, either a chipped p28 with the j35 basemap or a j35 ecu with the matching key and basically remove the immobilizer in the steering column, rewire it down to the ecu or away from the steering column and tape the key to the immobilizer. This way your key won’t interfere with it and you can still start your car.
Or you can just install a starter switch to bypass the whole immobilizer. Next up you’ll need new axles if you don’t already have a v6. Your hood might not close because of the extra displacement. That’s just a hunch though. Not sure how much more .5 liters is going to make over the stock j30 but afaik the j30 barely fit into the engine bay to begin with. Everything in the engine bay needs to be switched around but that’s baby stuff and take 5 minutes. Your power steering line might not reach since the pump is on the opposite side. You’ll either have to delete it or swap out the high pressure line.
You’ll need new radiator hoses if you plan on keeping the radiator you have now. Plus the lines from the heater core will have to be extended. You’ll probably have to upgrade the brakes if you plan on doing pulls to disc all around like the v6 had. There’s probably some other stuff I’m missing but that’s what I can see being the most annoying to swap over.