Never heard of coolant coming from the outside of 4.0l heads.
Lower intake gasket is a common leak point, where intake meets the head at the front or back where coolant passages are located.
Core plugs(freeze plugs) can also leak coolant.
The total job cost should be under $300 depending on what you have done to the heads
If you take them off then they should be cleaned and surfaced which is a few days and dollars at machine shop, you need to call for price.
They can also install new valve guide seals for you if you don't have a valve spring compressor tool.
Valve guide seals come with head gasket set.
New set of head bolts is needed
Rest is just wrench work, take lots of pictures, they are free
Any Regular coolant(green) is fine to use, but run water for first start up to make sure there are no leaks.
Newer vehicles are using OAT(organic acid) coolant now(red) it lasts longer, 5 years, compared to 2 years on older silicate(green) coolant.
And yes they shouldn't be mixed, they lose corrosion protection if they are mixed, engine won't blow up or anything.
There is a crossover coolant type called HOAT(hybrid organic acid) that can be used if engine was previously using "green" and you want longer lasting coolant, or you aren't sure which one to use.
One is not "better" that the other it just lasts longer, and so costs more, lol.
And you can't tell coolant by color anymore, Green and Red where used originally but everyone seems to have there own color code now
So read the label, silicate base is older style, OAT is newer, HOAT newest