Any advice on the oil?

Which engine do you have in your Jimny? Check the valve clearances if you're concerned.

Oil is supposed to get hot. The air under your bonnet will also get hot. Neither are a problem. Also 4krpm is nothing, they will sit at 5k all day long. And they will go around corners without oil starvation. Look at the sump design. The pickup is in that deep bowl at the back. There's no way it's getting starved of oil.
I've raced mine on an oval flat track without any issues. I've also done ~400 mile trips at 85mph without any issues. And I regularly see 6,000rpm in 3rd gear to maintain 60mph towing my 1,200kg trailer up hills, again no issues.

Don't over think it! Do your rallying and learn as you go. You'll soon realise that you should have put this effort into the suspension and drivetrain, and not worried about the engine.
 

the jimny is a 2000 model has 144k kms, but the engine (non-VVT M13A 5 speed manual) was rebuilt (i think) 6 years ago because my gran (it was her car) ran it with no oil and grenaded the engine, i don't know if the engine was swapped or if they just resleeved the cylinders, my gran doesn't remember and to be honest that mechanic was an idiot and did more harm than good to the poor thing.
also yeah i should have the valve clearances checked, my new mechanic told me that it could be the problem (and i think so too since they haven't been checked in forever).
also, right now the car is sitting in the shop because they're swapping my old dampeners for reinforced offroad ones and repairing the steering box since it's almost completely broken.
as for the oil, would an oil cooler be useful? and i'll make some vents in the hood i guess, i wanted to keep it looking as stock as possible on the outside but form comes second IMO

I stand with what Busta said
Put a lot more time and money into suspension set up
Something like a Dave Marsh Racing jimny kit 
Just push the engine till it blows...........you will be surprised how much abuse the M series will take 

well ok then, i'll push it till it can't push anymore responsibly obviously
and i'll check that racing kit, thanks for all the help guys

Sounds like your problems are to do with the engine's previous history. As Jack says, push it until it gives up. Then fit a good engine. Go for a 1.6 if you have Suzuki Lianas in Italy. They should be cheap and it's an easy way to gain power. An original engine with regular oil in it, serviced according to the service schedule will do everything you want.
You won't gain anything from putting holes in the bonnet, or fitting an oil cooler. Spend money on things that will make it a better rally car, such as a Quaife diff and the lightest wheels you can find.

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