A timing chain isn't like a timing belt...You don't have to replace it on a scheduled interval. You replace it when it starts to make noise (or, gulp, fails). The earlier 4.0 SOHC had some timing chain issues but they've been more or less resolved in the later models. Since the chain is tensioned by oil pressure, it's common to hear a little chain rattle when you first start the engine, but mine has been doing this off and on since I've owned it, and it never has given me a lick of trouble. Interestingly, since I've stopped using Motorcraft oil filters, the chain rattle on a cold start has all but vanished. I find it kind of ironic that in my owner's manual, it cautions that if an aftermarket brand of oil filter is used, that more start-up rattle may be experienced. That was the one thing that reduced it on mine! I've been using Baldwin filters for some time now, and they've been great.
As cheap as these engines are used in the salvage yard, I'm not worried...If I cannonball it tomorrow, I can get a replacement engine with 16,000 miles on it for $750.