Considering you probably paid close to $23,000 for that tractor when new:
Deductions:
15% right off top - $3450
$10 per hour - $1500
No warranty and 4 yr old - $1000
That leaves you at about $17,000, which is a fair price, but you may have a hard time selling it at that price because you have to compete with dealers who can offer financing and other services on a similar tractor. I think I'd price the tractor high since you say it's in very good shape, maybe $17,900 or something similar. If you get someone to pay over $18,000 I'd be very surprised. At those prices you start to get way too much competition from dealers. Good luck!