So I'm buying a HITEC x4 charger, what gauge charge leadbwires are good to use?

I've seen 12g, 14g and 15g

Does it matter?

14g or 16g will work fine. Anything bigger is overkill, IMO.

Unless your charging at 25 amps+, 14 guage is plenty. I would be sure to use quality high-strand count silicone wire for the best flexibility and durability at any temperature. Deans wet noodle wire is very high quality wire.

So the 12 gauge that the HITEC x4 comes with isn't good go with the 14 gauge instead?

I'm sure the stock wire it comes with is just fine. You do know that as the gauge goes down, 14g vs. 12g, the wire size actually goes up right?

Yes, I just wanted to make sure a bigger gauge won't harm the charger or batteries, since my batteries are 12 gauge now. I can never be to careful when lipos are involved.

Charge leads are not normally an issue. Balance leads can be, I know my Hyperions are calibrated for a certain length and gauge wire for the balance leads and using something outside of those specs can cause issues.