is 450w psu enough to use msi gtx 970 4gb gamingg 4g?

It is certainly recommended to have a good tier 1 or 2 550W PSU to run the GTX 970. 550W will give you a nice cushion between your system's max draw and what your PSU can handle.

If you have a high-quality 450W PSU you would probably be able to get by as long as you don't overclock your system at all. We would need to know the exact model of your PSU to be able to let you know if your 450W would be ok.