first off, your question(s) have nothing to do with PC power supplies, you should have posted this in the car audio section. but to answer your questions; the short answers are:
1. no, that connector is not suitable for the job you wish to use it for.
2. no, 4awg wire will not fit into the SB50 connector.
the long answers are:
1. your amplifier has two 30A fuses, that means that it can draw up to ~60A at full load. you wiring, connectors, and main power fuse should be capable of handling at least 60A. the SB50 connector does not meet this 60A requirement.
however... if you never turn it up loud then it won't draw as much current so you could get away with using wire and connectors rated for less current as long as the main power fuse is rated for no more current than the weakest component can handle (example, if your wire can handle 120A but the SB50 connector can only handle 50A, then that is the weakest link. this mean that your entire system is only good for up to 50A, so your main power fuse should be no more than 50A. this will be safe, but if your amp draws more than 50A, then you'll be replacing alot of fuses )
2. the 50A anderson SB50 connector is designed for up to 6awg wire, if you're going to use 4awg wire then you should use the 175A anderson SB175 connector. if you insist on using a connector for your power/ground wires then this would be a better option. this way you don't have to worry about using a connector that cannot handle the current that your amplifier may require.
also, I would not recommend putting an amplifier inside a subwoofer box. amplifiers get hot, they need air to circulate around them to carry the heat away. if you leave it inside a sealed box then there is no place for the heat to go.