If the main board burned out, there is good chance power supply is damaged besides capacitors, reason being whatever damaging voltage went to the main board, had to come out from the power supply, so you have very good reason to replace both. As a matter of fact there is always a chance something else could be damaged, besides those 2. Really a shame. If I was you I would keep all the receipts, things like that are prime example why class action suits were invented.
It's really hard to judge here, I'm sure Samsung wasn't aware of substandard capacitors at the time of assembly and no company can afford to support their products for free forever, on the other hand expensive TV should not fail after just 3-4 years, especially en mass, like it's case here. I just wish you good luck, hopefully all will work out well and this repair is not too expensive.