I have towed all kinds of stuff behind my XJ. The heaviest being a boat that weighed 5,000 pounds, which i towed over Donner pass on hwy 80. Trailer had surge brakes, my Jeep has and LS motor as well as frame stiffening, brakes are all upgraded and the rear axle is an 8.8. Accelerating and stopping arent the issue, its the weight behind you telling you who is in charge. Honestly, anything over 3,500 is pushing it in an XJ, and MJ would probably be the same. At 4,000 pounds with yours would be a tad bit on the unsafe side but doable for one trip.
I regularly tow a 2,000 pound trailer long distances and that seems to be a safe load for the XJ, so if you dolly it (which is tough on a 914) you should be close to that weight.