Okay, assuming your voltage is constant at 220V, and your main breaker is rated 40A - that gives you a theoretical maximum of 8800W, or 8.8kW load, before the fuse breaks.
The "designer" such as there will have been one, has relied on something called diversity, which as the simplest of expressions is saying not everything is likely to be switched on at the same time, and you are unlikely to need to draw maximum load. I guess that "designer" also didn't consider the advent of electric showers which would swallow all but maximum available load either.
One of the potential issues here is that your maximum load on the 20A breaks is around 3.3kW - so in theory, you *could* be calling for a demand of nearly 12kW - 55A at 220V.
It is, I suspect, fair to say that Thailand does not have any hard and fast regulations regarding electricity, although things are improving. Notwithstanding, cable and electricity work in the same way wherever in the world you are.
Your cables are designed to carry a specific maximum current, based on many characteristics, including efficiency, heat dissipation, insulation characteristics and more. Where those capabilities are regularly exceeded, the cable will break down much more quickly, as it is exceeding it's thermal operational properties, it will get hot, and then brittle, and then break. As it gets hot, the insulation resistance reduces, and once it gets brittle, the insulation breaks away, leading to shorts, in turn leading to secondary fire risk - the first being from excess heat in the cable it self.
Again, in THEORY.....assuming the main breaker has sufficient tolerance to allow 55A through without tripping/breaking, you are still going to do some damage over time. This is one reason why, in the UK, supply cables are rated for maximum demand, and diversity is only applied to certain final circuits.
In THEORY......you *could* use a 4mm CSA cable to carry 55A of load, and given the situation, if you then theoretically applied some kind of diversity, you would arrive at a conclusion whereby you would switch everything else off while using the shower. The problem really comes when you move out and the next person doesn't know to do that, and then runs everything at the same time. But you are still overloading the cable every time you run the shower.
You'd be better off, if at all possible, taking a 10mm CSA minimum from your supply to your main switch, which should be uprated to 55A. 10mm CSA will offer a maximum current capacity of around 57A enclosed in trunking, or around 65A if clipped direct.
4mm CSA cable is only rated for 32A when enclosed in trunking, and around 37A clipped direct - so is under-rated for the supply available.
The risks, essentially are as you describe them - fire, insulation break down, and potential shock, failure of the equipment being fed, and such - not to mention a waste of money if all you end up doing is replacing worn out cable.
