Thanks very much for the input, actually there has been no movement where the tank would go as far as I can tell, it is the high spot while the other corner is the part that has settled, with a crack running down the middle but not under the tank spot. I'll try to find the slab thickness and update you guys! Thanks!4 tonne on a concrete slab is not considered exessive at all but it does depend on the surface area and whether the slab was made to building code.
You mention there has been slab movement where the tank will go? A structural inspection would involve soil tests/drilling as well as the thickness of concrete. And if deemed insufficient you would then have to reinforce the slab.
I would reinforce the slab instead. Might as well save the inspection fee and get more peace of mind.