Do you mean extension for power cord to psu or extensions for psu to gpu cards?

If main power either buy a longer cord or buy an extension cord thst can handle the power draw.  In this case at least 750w but i would go higher.  If 110v power then a 15amp cord.  If 208-240v then a 7.5-10amp cord.

If you mean for psu to gpu then you can get extensions but for 4 gpus should be needed.  i run 6x13 gpus off the psu depending on size.

Just be sure to get good quality wire and made ones.

Risers come with 2-3 ft usb cable so shouldnt need extension there.