If it's an original V4, then you'll want to ask the current owner if it's been modded in any way.
Then plug it up and listen to it--play at low volume and see if you get any overly loud hum or buzz.
Be sure and check the rocker switches, as they sometimes get gunked up. It's a relatively easy fix, but like pots they can become dirty.
Check the fuse and make sure it's the correct one for the amp. If a guy replaced the fuse with a significantly different one, bad things can happen to an amp.
If you can, open the amp up and have a look inside--even if you're not an electronics expert you can often identify issues just by looking. If a guy spilled a full beer inside the amp, you'll know it!
The biggest potential expense would be a faulty transformer, whether power or output. There's not a lot you can check for visually, but look for large amounts of rust on the surface of the transformers--that can be a bad sign.
Look for scorch marks inside the tubes, around the tube sockets, or anywhere on the PCBs. This is usually the result of arcs and can mean trouble.
Lastly, look for bulging or oozing capacitors. This is a relatively cheap and easy fix, but left undealt with can mean a catastrophic failure. Here's a pic of what one might look like:
Mostly, just play the amp and see how it sounds. They're great amps and I hope you find a winner!
