Reading, reading, and more reading, isn’t a bad place to start…unfortunately this place really doesn’t have much in the way of info for beginners…and much of the info that was available on the web 10 years ago has been removed.
Pressure testing is a solid first move. However, I recommend to hydro testing rather than using compressed gas.
See eg @Killa12345’s recent account: Safety Lesson: How being lazy nearly cost me my life! Extractor Accident
There are almost certainly links or descriptions of hydrotesting there.
4bar isn’t an unreasonable working pressure, and your system probably won’t see more than that in use. However, testing the pressure relief valves (prv) is also important, so taking it up to 10bar (or wherever they are set to pop) is advisable.
I don’t see a prv on your solvent tank, but there is a port we can’t see, and I’m hopeful that the pressure gauge is obscuring it. If not, you need to install one.
As @Killa12345 mentioned, solvent in the freezer is one of those Bad IdeaTM’s…butane is heavier than air and will fill your freezer from the bottom. If there is a vent at the bottom (fairly standard, to let water out), it will find its way to a source of ignition (closest being the compressor).
Yep, those Chinese valves are horrible, they may not even have adjustable packing nuts. Paying a little more for decent replacements is probably your next step.
Not sure you really have much in the way of options for salvaging the solvent you’ve loaded into your tank, you may need to vent that to atmosphere