2 10 inch lanzar 2000 watt subs should you buy a 2000 watt amp or a 4000?

Definately not the 2000W amp - it will have to work so hard to drive those subs that you'll probably kill their voice coils due to excessive distortion. Some people subscribe to the "matched" concept. EG: 4000W of sub needs 4000W of amp. My personal rule of thumb is that your amp should be 40% more powerful than the subs. This allows you to set the gain on the amp lower, thereby minimizing the afforementioned distortion. Given that you have 4000W of subs, my rule of thumb would dictate a 5600W amplifier. There's no need to be that precise though, a 5000W amp will be quite sufficient.