4.0l OHV was made from 1990 to 2000, in 2001 Rangers got the new 4.0l SOHC, these two engines are not related.
It is an interference engine and uses a timing chain, but on the 4.0l OHV engine these tend to last the life of the engine, so unless other engine work is needed there is no reason to change the timing chain.
4.0l tends to crack heads if overheated, so that should be avoided, i.e. do not try to "make if home" if engine starts to overheat, stop engine and let it cool down, $10 rad hose break could cost you hundreds.
Water pump will usually start leaking out of it's weep hole before it fails but up to you if you want to replace it.
4.0l uses a 192 or 195degF thermostat do not use a 180degF, and make sure it has a "jiggle valve"(hole to let air out)
Fuel filter, $10, is in the frame rail under the drivers seat area, good to replace these now and then.
MAF(mass air flow) sensor on the air cleaner housing should also be cleaned now and then, not hard to do, this is the main sensor the computer uses to set air/fuel mixture.
If it gets a little dirty you will get bogging and stalling.
The heater core on the '94 4.0l is the cooling system bypass, so coolant always flows thru the core, if you notice engine temp starts to fluctuate for no reason then heater core is probably getting clogged up, $28, easy to change, this will not overheat engine, temp needle will just go up above 1/2 then back down to below 1/2, RPM, uphill, downhill, doesn't matter, it will be totally random.
Next time you change the coolant, backflush the heater core, it is easy and everyone should do this on any vehicle.
Over all the 4.0l OHV is a very reliable engine, I am above 300K and outside of my learning experience with the "do NOT drive home with hot engine" I have no complaints.
The automatic in the '94 should be the A4LD
"The A4LD four speed overdrive transmission was introduced in the 1985 Ford Ranger and saw use behind many 4 and 6 cylinder engines until the 1995 model year when it was replaced by the 4R44E and 4R55E electronic transmissions. The A4LD is identified by transmission code 'T'."
Info on the A4LD here: Ford Ranger A4LD Automatic Transmission
You have "auto hubs" for front axle on the '94 and the "electric shift motor" to activate the transfer case.
Best thing you can do is to shift it into 4x4 once a month, this keeps the electric motor's brushes clean, these tend to get stuck if not used for a long time, so when you do need 4x4 it won't shift, a few taps with a hammer will usually get it working again but just for a bit, it will need to be serviced(cleaned).
Many have replaced the auto hubs with manual hubs, my '94 auto hubs still work fine, just do not over grease them, they need to be able to move to work.
You can check the Axle code on the door sticker, you probably have a Limited Slip rear axle, most 4.0l 4x4s had that.
Look here: Ford 7.5 & 8.8 Inch Axle Tag & Door Codes
It will also tell you the ratio of both rear and front axles