You have 1 12 inch 4 ohm 400 watt rms sub and an amp that will push out 500 watts rms at 2ohms and 300 watts rms at 4 ohms will this amp be sufficent enough for the sub?

If your subs are 4ohms each, then yes it'll be easy. Hook them up in parallel and you're up and running at 800W! :) I would recommend that you purchase an amp capable of more than 800W x1 @ 2ohms though. My basic rule of thumb is that the amp should be approximately 30% more powerful than the speaker(s). This is to help ensure that you're not feeding the speakers distorted sound. Distortion is the enemy of voice coils(and usually the cause of premature speaker failure). So, 800 x 1.3 = 1040. I would recommend an amp capable of at least 1000W x1 @2ohms.