DOT 4 spec has a higher boiling point than DOT 3 and can be used where DOT 3 is called for. It is usually spec'd for use with disc brakes due to the direct contact of the brake pad to the caliper putting more heat on the fluid. If added to a system that already contains DOT 3 fluid then the slightly higher boiling point goes out the window. You can use either fluid and mix them if necessary without a problem. What you can't do is mix DOT 5 spec fluid with DOT 3 or 4.
Since you installed a new front system I'd take the time to completely flush the entire system with new clean fluid of whichever spec you intend to use.
Mike