I actually have 8gb ram ddr3 gtx 960(2g) and i3 4150
most of the time ue4 crashes or becomes slow (4.19) so should i modify graphic card or cpu ?
if cpu would i3 8100 do the job ?
Crashes might be lack of RAM, but you should upgrade your CPU first, but an i3 8100 isn’t much of an upgrade.
What about i5-8400 ?
you can find all this information on google 
http://cpu.userbenchmark.com/Compare/Intel-Core-i5-8400-vs-Intel-Core-i3-4150/3939vs2309