That price seems reasonable. I bought mine for $3,425 two years ago. With their known long life (as long as they are maintained) a 4 yr old stove is barely broken in.
I am no expert but I would check the following:
1) condition of the interior, especially excessive corrosion. If you see any fly ash laying around (I am assuming it is currently being stored and not used) than its maintenance may come into question. These stoves need to be wiped totally clean at the end of the season.
2) look down the exhaust tubes (through the 6" nipple you attach the stove pipe to) with flashlight, again checking for corrosion. These should also be brushed as clean as possible after each season.
3) check to make sure the burn plates aren't warped
4) check the stoker hardware. You may be able to get pictures off these forums showing you what they should look like.
5) check the coal bin for corrosion. This sometimes is a problem area, again due to poor maintenance. Coal can't be left in them all summer long, they need to be cleaned up just like the stove.
6) You could try out the motors (fans), but being only 4 yrs old, they should be ok and should give you a number of more years of service.
Stay tuned, others with more years of knowledge will probably be able to add to this.