What tool merges DEMs?

I want to merge the DEMs for 4 adjacent counties. They don't look good as is because the high point of one can be the low point of another. I used to do this in Raster Calculator using Merge, but that's not available anymore.

Mosaic to New Raster gives me a result with high and low values of 255, not very helpful. It's been a while since I worked with DEMs. Is there another tool I should try? Thanks.