I have a 5 gang metal switch box with 5 switches. I want to eliminate one of the switches so I have only 4 switches. I don't want to use a blank to fill the hole with no switch. I want to use a switch plate for 4 switches. Can I cover the hole with a metal plate and mud over it so it looks like a 4 gang switch box?

Thank you in advance

You might be able to remove 5 gang MR, and install 4 Gang MR, as both may? Use the same box, many do, unless you have the larger box, which may handle 5 + 6 gang.

Since you are going to be mudding the wall any way, make enough room to see if you can't just remove the last box, take the end plate off it and put on box number four. Should be a screw top and bottom ganging each box together.

Is this Gangable boxes like hk mentions, or one large mud ring?

It's one large 5 gang box. Do they make a 4 gang mudring (4 switches) that will fit on a 5 gang box?

If you have a solid 5 gang box, not the typical 5 wall cases joined together as explained by Hkstroud, then this is what your looking for,

See "Gang Box Adaptor Covers" at the bottom of :Welcome to Arc-Co Electrical Boxes and Fittings

A picture of the box you have would help eliminate the guessing.

Thanks tkrussell,
As always you have an answer. This is exactly what I need. Now I need to find a store that sells one...

Real handy site from TK, You may not need to special order.
Measure width of box, take that measurement to supply house. They may have the 4 gang mudring that works with it. Be sure you match the depth, Many Supply houses down here have 5/8" raised in stock, but have to order the 1/2" raised in stock? Would think
It would be the other way around.