I installed new pistons after having engine bored to specs. I ran engine for a few minutes then removed oil pan to do inspection. Number 4 piston appears to have some minor marks on both sides indicting a slap. What did I do wrong?

First I would guess (with out see the marks) they are normal. Piston slap would indicate the pistons are too loose. That should not cause marks. If the pistons are too tight (when hot) - that would cause marks. If you can't feel the marks with your finger nail I would say they are just beginning to wear in (break-in). My thoughts

Thanks

Scoring on both sides may indicate that the piston is too tight in the bore.
As you don't have scoring on the other pistons I presume the block was well cleaned after boring. Most machine shops regard their job completed when they have bored a hole, honed it and wiped the surface clean with a rag. Honing grit is almost always imbeded in the cylinder walls. The only way to remove it is with soap, water, a scrub brush and a lot of scrubbing.

Did you install new piston pin's, when you did your rebuild?
This maybe a factor if you used old one's with new piston's.
Old pin's could be letting your piston,s to tip a bit, causing drag on the piston well's. (check your clearances on your pin's).
Also check for crank trueness, a sprung crank does not help.

Keeping vintage Chevy alive.