Is there anyway you can increase the magazine capacity of a MRC 458 Lott from 3 to 4? Does anyone make different bottom metal?

Good question; wish I knew the answer but if it is indeed a M-70 clone and the hole spacing matches, I would think any Winchester M-70 bottom metal from Sunny Hill or CNC would work.

Harry,
that's what billy did on his 404... sunnyhill dropbox... LOTS of money for that extra round, though, new stock and metal
jeffe

I am guessing from Jeffeosso's comments this would require extensive stock work?
My rifle is built with a McMillan stock.

PS. I should of just bought the LH Dakota action as it holds 4 down

Yup. The box hangs below the stockline. On a synthetic that can be handled by building up the bottom and repainting. Impossible to do on a wood stock and still look like anything good.

The only other option and not a cheap one would be a one-of-a-kind pocket floorplate like Jim Wisner makes for the CZ 550 magnum.