Basically you want to match the subs in impedance and power.
With 4 ohm subs wired in parallel, the final load will be 2 ohms. Depending on how hard you want them to hit, a pair of 400 watt RMS subs will need an amp that delivers 600-1,000 watts into a 2 ohm load.
The enclosure you use will also make a big difference. A sealed enclosure will need more power. A ported enclosure will be louder with less power.
The Hifonics shown here would be a good choice for a sealed enclosure. It's rated for 1,100 watts RMS into a 2 ohm load.
The Eclipse XA1200 outputs 600 watts RMS into a 2 ohm load. It would power the subs very well in a ported box.
Somewhat less expensive but good quality would be one of the Alpine MRP series. The MRP-M500 is only $179.99 at SonicElectronix. It delivers 500 watts. It's big brother is the MRP-M1000 with twice the power. Be sure your subs are securely mounted or this amp will literally "lift the subs".
Hope this helps.