Any good way of using some 16:9 footage to a project shot in 4:3? Client has given me some clips in 16:9

Letterbox (black bars top and bottom) or zoom (cut off the left and the right) with panning as needed ("pan & scan") to keep the action well framed.

In general, people pan & scan in narrative works to keep the video from drawing attention to itself. The exception is when there is critical information on the edges, like in many credit reels.

What do you edit with Darryn?

I am a PC and edit with Cyberlinks Powerdirector 11 and also Corel Videostudio.

switching between aspect ratios within the same project can be a bit annoying for the viewer.

that leaves you to make the decision whether to keep the project 4:3 and pan and scan the 16:9 stuff. or switch the project to 16:9 and crop off the 4:3 footage to become widescreen.

which would damage each respective footage more: cropping the sides of the 16:9 footage, or cropping the top/bottom of the 4:3 footage

since we're living in widescreen world now, i'd lean toward making the project 16:9