Can I use # 4 awg thhn copper for 100 amps is it big enough ?

Use #2awg copper minimum at least in Canada, assume US to be same.

#3 THHN copper is rated 100 amps, as per Table 310.16 of the 2008 edition National Electric Code for a feeder.

If this feeder is the service entrance conductors for the service of a dwelling, you may use #4 copper.