Its not necessary but to get the most out of the subs you'll need two amps. You can do more harm to the subs with an underpowered amp than you can by having too much power.
I'd try it with one first, and if you are satisifed with the volume and they're not distorting when you're pushing the amp hard then theres no problem. But it will definitely sound better with two amps.
Make sure you wire the subs correctly to get the right impedance too. 4 subs wired in series are going to create a savage impedance, probably drop to about 1 ohm. If you do it this way make sure the amp can handle the load.
Actually, it depends on what your set up is. Underpowering any sub will never hurt it. Trust me, i've been around the audio scene for a long time. You can even go to forums with audio gurus and they will tell you this. If you are running a sealed enclosure, overpowering it isn't going to hurt the sub. It would be better. If you are running ported, you usually can underpower it. Overpowering a ported sub is bad...usually blows. Getting two amps isn't going to make the subs "sound better". Just make sure your subs are getting proper power. And the most important thing of getting good sound is the enclosure. I work at an audio place all day long so i know these things. I delt with He2's before. They are 400 rms, 800 max. You don't go by the 800. You always go with the RMS(continious watts). I recommend going with a fosgate bx1500 amp. Puts out around 1600 watts and comes with a birthsheet of it. It should power them nicely. Getting one amp is also easier on your electrical system also
Hope that helps a little..i just wanted to clear some things up with veilside300..