4 year old diesel---pitch it? or use it?

Keep any old diesel in an oil drain pan. Use it to clean the greasy areas that you don't want to shoot 3500psi water at.

 

With that stuff in tank drain it. I use Zep ring master acid toilet bowl cleaner. Works on aluminum and stainless steel. Pour in a gallon pure tank and slowly fill to top with water. Let sit for a couple days then stick garden hose in the filler neck and let it back flush. Then pull drain and flush. Every aluminum tank i’ve cleaned out this way has come out almost like new.

 

Yes. Great idea. You can also put it in a spray bottle to use as a degreaser.

 

That stuff looks nasty . On your drive if possible every 30 minutes or so I’d stop and check for seal leaks . If it makes the 2 hour drive your probably good for longer trips.