48" x16"x 8/4 Slab of Walnut..Can I relief cut the bottom to take out the sag?

The effect of such cuts will be hard to predict. Better, in general, to
screw the board to stretchers of wood or metal underneath to pull it flat.

Kinda what I was thinking….. Thanks for the reply.. I was hoping somebody may have tried this already and could share their “success story”

You can rip a 1/4” strip off one edge and see what it does when you remove
some material from the side you’re thinking of. If it bends the way you
want, chances are the whole board will do the same thing, more or less.

There’s a science to studying growth rings and anticipating wood movement
based on this. You get an intuitive sense of it with experience, but I’m
still reluctant to take the kind of chance you’re considering.

If you put 2 saw kerfs in the bottom running lengthwise, you can fit one
leg of pieces of aluminum or steel angle iron into the kerfs and screw the
other leg to the bottom of the board. This way the reinforcement is
barely noticeable.