I own an '02 (2wd) and a '10 XLT (4wd), each with 4.0 SOHC V6.
I bought the '02 (169k miles) with a bad engine for $500, and swapped a used engine (116k miles) in. Had to replace both front wheel hubs/bearings, front struts, and lots more. Developed trans issues around 180k, which I fixed with the kit that replaced the Overdrive Piston (from Wisconsin). (I also had a '97 Explorer, which me son rolled and we won't talk about.)
The 2010 runs like a dream. However, I bought it after the previous owner had the trans rebuilt (116k miles). I even spoke with the shop to make sure they replaced the torque converter with new and bored out/sleeved the OD and 3rd gear piston bores. The mileage is worse (due to 4WD), but gotta love that 4WD high in Michigan winters, and the ability to tow a boat out of a lake or tackle a steep incline with the 4WD low. You wouldn't get that on the Mountaineer AWD.
Hard to find V8 4th Gen Explorers. There must be 6 or 7 of the SOHCs to every V8 out there.
In short, that 4th gens built were a refinement to a very established truck that Ford had been building for years. I don't believe that they are any less reliable than the 3rd gens. If anything, they may have beefed up either the front or rear hubs/bearings, which were failure prone. At least Rockauto shows different parts numbers for one or the other. The fronts aren't too tough to replace, but the backs are a bear . . . .
Other than that, Ford has a history of discrete, unanticipated faults in the first model year or two of a new model. So the '06 and early '07 Explorers had some radiator issues and maybe some drive-by-wire anomalies that Ford eventually worked out.
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p.s. Tons of info in the 3rd gen forum on the timing cassette issues on the 4.0 SOHC. I've done that job twice; very tricky, and need a few special tools. Good maintenance (oil changes) with high quality filter (Motorcraft or Wix) is key.