there are 4 servers with the same hardware configurations, and a ipvs based lvs system is constructed on these 4 servers using NAT. my question is simple: will there be very high load on the front server which was in charge of transferring traffic to the whole lvs system?
To be clear: You're setting up a single server to act as load balancer, using Linux' IPVS kernel module? And the load balancer has around 4 or so backend HTTP servers behind it?
No, that shouldn't be a problem at all. IPVS does load balancing at the TCP/IP level, so it's capable of very high packet rates on modest servers. Handling 4 backend servers should be no problem.