I went through this same dilemma when I decided to add a Jeep to vehicles I drive. I thought
I would buy used and prior to a JK but the more I researched the more I discovered Jeeps depreciate slowly [due to demand being so strong and supply being limited]. I had decided I'm always going to have a Jeep so resale wasn't a factor to me. You've already narrowed your search to 4dr JKs [try throwing all Wranglers in the mix and do the opportunities really widen
]. I discovered that Jeeps depreciate about $1k a year on average. A 10 yr old Jeep will be about $10k less than a new Jeep. It may have a bunch of after market good stuff on it, adding a bit of value, it may be beat to shreds [not knocking much value off of it]. You'll be buying mileage, and possible headaches, used.
I decided to buy new.
What I did, and what I recommend, first read and read about 4dr JKs [since that is your target] and what options you want for it. Join Tread Lightly to lock in the discount. With the options you want, price how much it would be to buy new with the stuff you want Chrysler to put on it.
Opinions differ but it seems to me that the general consensus is the 3.6 engine is at least as good as and perhaps better than the 3.8 [another reason to look at '12 and newer] and few dispute that the new auto tranny's are better [again '12 and newer].
I don't believe the amount you save between recent/used and new will be less than the value you'll lose buying recent/used v. new. You don't have to compromise w/ options on new, you have a warranty, you'll pay a few thousand more for a more valuable vehicle: in a few years whatever you buy is going to depreciate at about the same $1k a year.