Hi Mike, welcome to explorer4x4.com
Nice to see someone from my neck of the woods. I see you live in Leawood, well, I live in Mission, just off I-35 and Lamar. Welcome neighbor.
Hopping up the OHV 4.0 is definitely an option to swapping in a V8. Vanir makes some parts, as well as 4.0 Liter Performance, a division of Central Coast Mustang. I checked into that before deciding to go the 5.0 V8 swap route. My take was that Vanir was very proud of their products (I mean very $$$$$). Of course I think their prices are high because there isn't much competition in 4.0 aftermarket performance parts. The opposite is true in the 5.0 V8 market - there are literally thousands of vendors to choose from, which creates good old competition for your dollar, which in turn is good for the end consumer - you and me. Another plus on the 5.0 side is the old line, there's no replacement for displacement.
You can make some decent HP out of the 4.0 though, with the greatest gain coming from a blower addition. This, along with a new cam, can really wake up the 4.0. There are some options on the intake side, like larger MAS and throttle body. I could be wrong, but I don't think there are any aftermarket intakes available. The stock intake can be massaged a little, but that's rather on the esoteric side, for the money you spend, I don't think you'll see much performance increase. Havn't checked lately, but don't know of any performance heads available, although some work can be done to the stock ones, although I'd probably go with a new, updated casting - the old heads have some inherent weaknesses in their design, and are prone to cracking over time. I know that personally, my driver's side head gave way recently, it had been cracked for some time. Not uncommon for this to happen on older 4.0's with high mileage.
On the exhaust side, there are headers available, and of course, a performance muffler and possibly a high flow cat will net some HP.
If I were to hop up a 4.0, I'd probably save the money that it would take to buy performance pistons, roller rockers, port and polish intake, etc., and put that money into a blower, and a performance cam to go with it. Dollar for dollar, that's where you're going to get the most bang for your buck. BBK sells the Eaton roots style blower, and I believe you can also go the centrifugal route with Powerdyne I think. Talk to Dennis Hilliard of Central Coast Mustang, he's built a blown 4.0 2wd Ranger that cooks - he's taken it into the 12's. Competition Cams makes a couple different grinds for the 4.0, and I think there are few other possibilites out there. Also, with the blower and cam, you'd probably want to upgrade the fuel system with a larger capacity pump and injectors. More air in without adequate fuel doesn't work too well.
Keep in mind that whatever you do to get more power, the stock transmissions in the explorer weren't built to stand very much HP. My stock 5 speed has served me well, and it can be upgraded somewhat (heavy duty clutch, throwout bearing, and slave cylinder, but it still ain't no T-5 or Tremec), but I hear from many that the automatic is definitely a weak link. With my 5.0, I'm going with a performance AOD that can handle all the power I can give it. Don't believe you said which tranny you've got. Something to think about........
When you say you want to hop up the 4.0 instead of spend thousands on the swap, realize that to get substantial gains out of the 4.0, you're going to be spending "thousands", at least 2 or 3. Now this is cheaper than the V8 swap, but it's not exactly pocket change. But then again, I'm not sure of how much HP you're looking for.
Talk to Alec on this site, he just installed a blower. There are some extensive threads covering the installation of his power adder, on the Modified Explorations forum.
You've come to right place to find out everything about Explorers. I'd do as much research as possible before spending any money. Good luck.
