How hard is it to convert to a 4.0 from a 3.0 in a 2004 ranger edge 4x4

 

Very hard compared to other engine swaps.

3.0l and 4.0l are not related so transmission will need to be changed, so maybe drive shafts as well

In any engine swap you have to change computer and wiring harness, and since your year also has PATS(anti-theft system) you would need to get a dealer or lock smith to add existing keys to new computer, if it had PATS too.

All in all it depends on what you have now and what you want as the end result.
If current 3.0l Ranger is in good condition, money is better spent selling that one and buying a 4.0l Ranger.

It 3.0l Ranger has a failing engine then a rebuild or used 3.0l engine would be best bet

And just FYI vehicles without stock engine size, even if it's an improvement, are worth less money because they are a pain to work on, often parts won't match when you want to replace alternator or water pump, even spark plugs, lol, unless you know the year of the Engine

 

If you want to do an engine swap, a 4.0 stock is not gonna be that much better than a 3.0. If you're gonna do all that work you might as well put a v8 in there.

 

What he said... X2 !

With all the changes needed for either change, 4.0 or 5.0, might as well get the most bang for the buck.
With the 5.0 you might even get and increase in MPG but the 4.0 will go from 3.0 bad to 4.0 worse in mpg.

My '88 Ranger got better mpg with the '93 5.0 that replaced the 2.9L.