Usually the MRF150s are sold as matched quads. How much they're really matched or need to be, I dont know. It's anyone's guess if all 4 blew or just one shorted.
Can you replace them yourself? Yes, but it's not easy unless you have a hot air reflow setup. The transistor tabs and PCB traces are very wide and will sink the heat from a small soldering iron. YOu'd be there all day trying to get the solder to flow unless you have a large iron. I've done this before with a Weller iron and it just takes patience.
But... once you replace the transistor, you will have to reset the resting current on it. All four if you replace all of them.
This is a neat site where a guy built one himself and he documents what he did. It's good reading if you want to understand how that amp works. I think he might talk about setting the bias too.http://www.k0gkd.com/ssamp.html