We have a weird project where we have to mount 4 TV's on a wooden log in a restaurant. What do you guys suggest using for horizontal leveling?

How big are the TV's? How big is the log? You could always use a single stud articulating mount

What sort of bracket are you using?

You'll probably have a dickens of a time drilling bracket mounting holes in exact locations. If your bracket does not have any fine vertical adjustment capability, mount one or more plywood, 2x4, or metal plates to the log, then use a laser line to layout bracket mounting points on the plates.

If the TV's are close together you'll want them to be parallel. Likely you'll want the screens to be in the same plain too. "Level" is usually more of an optical alignment, rather than being exactly level. Shadows and non parallel nearby surfaces sometimes cause exactly level sets to look out of alignment.

If the TVs and mounts are the same for each, you could use something as simple as a string with a bubble level or a snap line- put a screw at the ends, pull the string very taut and make sure the string doesn't sag in the middle. If it does, add screws and make sure they align with the outer ones. Otherwise, you could buy a laser level if it makes sense to do that.

I'm seeing a big split log mounted horizontally with 4 TV's side by side in a line. If this is what you're talking about just use a laser level or snap a line and mount all 4 wall brackets at the same height. Use something that has "fine tuning" screws built in.

you will never achieve true level with bubble. ;-)
-Rick

We would need more info, but if they are suppose to be close together and of course depending on size you can try some video wall mounts, doubles, not sure if they have quads maybe two doubles spaced out a bit. Either make your own main back bracket or use spacers, we have boxes full of extra parts from hang and bang so spacers and stuff is not a prob for us. And you should use a laser. A log or a store any uneven surface will be a bit tricky but it's doable.

PK, so use a water level.