Found a crack in #4, in the middle of bore & not in any bolt holes......

Sleeve it, don't see why there would be an issue

I had it mag'd today, no crack....just a stain of some sort., engine guy didn't think there was enough material to sleeve it, close to bolt holes & in between cylinders.

Might be a pin hole. Pressure test the block. I had one once that had me chasing it until I found it.

I know your talking about a Merlin block.
For what it's worth I just sleeved a Dart block.
Darton sleeve suggested a 4.700 ductile iron sleeve.
Seems like it took forever to bore to 4.7.
Pressed it in and bored to 4.6.
Those ductile sleeves are very hard.
Had to cut it with my chop saw to get it close to cylinder length.

I have a dart block also getting sleeved and finish sized at 4.600 with no issues.. same as Bruce said above.. (this is a "new" build, not my crazy 533" lol)