problem with fuses

First of all, NEVER replace fuses with a larger size!  If the amp is supposed to have 25 amp fuses, then return it to 25 amp fuses or you may burn your car down.  Second, power wire size has nothing to do with how much current the amp is drawing so no, using 4 AWG is not causing the problem.  The problem is that your amp is either defective or you are putting too much load on it.  How are the speakers wired?  How did you set your gain?

But first and foremost, PUT THE RIGHT SIZE FUSES IN THERE or you are potentially sitting on a time bomb.

Sounds like a load problem to me.

The sizing of the fuse is only relevent to whether or not the fuse will blow before the wire catches on fire.

They are NOT current limiting or current controlling devices.  They are simply designed to open up of the wire is in danger of carrying more current then it can safely carry.  Fuse sizes, ideally, are selected based on the size of the wire being used, NOT the size of the amp.

Well, you absolutely must know the impedence of the sub.  It is a requirement.  It isn't a number that is nice to have, it is a number that is required.

Most subs will say right on the back of them what they are.  If you don't want to take them out of the box then you can disconnect the wiring from the amp and use an ohm meter to get a general idea.  Once you know the impedance of the subs then you can decide how to wire them.  Unfortunately, ceratain impedances and amps just are NOT compatible with each other.  You must pick the impedance of the sub based on how many subs you will be using and the minimum ohm load you can put on the amp.

You most likely have either 4 or 8, but 4 and 8 are night and different.  4s will work in some situations, other situations will require 8.  There is ALWAYS a 'best choice' when dealing with 2 subs.  Typically, if you have two subs you want 8 ohm.  This is because most amps are 2 ohm stable.

If you have two 8 ohm subs wired in parallel you'll have a 4 ohm total load.  If you bridge your amp with that load your amp will "see" 2 ohms.

If you have two 4 ohm subs wired in parallel you'll have a 2 ohm total load.  If you bridge your amp with that load your amp will "see" 1 ohm. 

The difference between 2 ohms and 1 ohm can be overheating, fire, fuses blowing, amps dying, subs blowing, etc. 

Again, knowing the exacft impedence is required here.