then i would use the current wire that you have available, and de-rate your main fuse accordingly(like i said, i run a similar setup-- the amps max draw is 140A but i installed a 80A main fuse, and have not had any problems so far).
note-- the right way would be to replace the 4ga wire with a 1/0 ga, and upgrade the other wires to the correct recommended size for the amps, though what i said above will work, but don't expect to push the system past 'really hard'....
power the speaker can take for many, many hours at a time(think running a speaker at full power for 24-48 hours at a time). i run nearly double the rms rating on all my speakers and have no issues in the 1-2 years that everything has been installed.
50w per ch should be just a hair quieter then your old amp--nothing that a slight tweak of the gain control couldn't fix. i wouldn't worry about it too much. but if it is a concern, get a more powerful amp(i only posted it because it was a model that worked out alright for me, though there are a ton of amps that would also work reliably that i haven't used).