4 PSI is way too much, I have stories about that, as well as dent removal. There is a readily available tool called a Mity-Vac, should be in ALL your sheds & shops, You can use for both pressure and vacuum testing and very utility-versatile. I think the chinese have copied them and offer at Harbor freight and other vendors as well.
https://www.harborfreight.com/mityvac-vacuum-pump-39522.html (This one seems to be suck, no blow, mine does both)
This one, is similar but still not like mine, but mine can easily switch between pressure and vacuum,,
https://www.grainger.com/product/MITYVAC-Automotive-Test-Kit-1XGR6Simply plug the petrol tap holes and fashion a stopper for the filler neck, I have some rubber ones from laboratory and industrial apps in different sizes and one I drilled/tapped and installed 2 fittings, one for pressure and one for the gauge. I also have a variety of corks, plugs and threaded and non threaded fittings, and save them as always useful so over the years amassed quite a collection.
I use a rubber maid large storage tub and while soapy water is useful for bubbles, immersing in a 20 gallon tub is better IMHO to spot a pin hole leak. Traditional radiator shops do this as well when soldering, brazing or welding a radiator or sealed vessel. If you dont want to bother, see if there is a local shop for a few Pesos $$$ see if they mind testing for you.
NEVER EVER try to use freezing water or Air Pressure to remove dents on a petrol/gas tank. It will not work out for you, is dangerous and will likely further damage or more likely destroy your tank.
I have a Norton C-Do tank on the wall down in the shop i use as a demonstrator of someone who should know better did so as well. I one tried it myself and was alarmed at just a few PSI how quickly the sides of the tank splayed out and distorted, needless to say the dent stayed the rest of the tank distended like the belly of the crewman on Sigorney Weavers space ship before the Alien popped out and we all know how that turned out.
So, I got a tank from a guy who, highly educated and skilled in his trade, Industrial Structural engineer (Engineers operate on their own special wavelength). He too tried the air pressure method and the NotRun tank splayed out like an elephant stepped on it. I often have used it to illustrate these discussions. I can snap a pix if you like????
The reason is simple,,,a dent ding or distortion in formed sheet metal distresses the metal and its grain structure. It stretches it and work hardens it as well. That means the dented metal is actually HARDER-STIFFER than the surrounding metal. Its the LAST thing that will budge.
Pulling dents is a fun & interesting skill and physics lesson in itself... not easily addressed in a short blurb, but happy to discuss, Shrinking, annealing and metal forming alloy or steel as well.