4.0l SOHC had a common leak issue from the plastic thermostat housing.
Coolant leak sources can be hard to trace, you can get a UV(ultra violet) dye kit(under $20), it has UV dye and small UV flashlight.
Run the dye in the coolant for a day(or two), then start trying to find the source while disassembling parts to get to it.
And keep in mind a leak can spray out, so where it is dripping may not be where it is leaking, those are a PITA to track down sometimes.
UV dye also works for oil leaks and even A/C leaks
Dye is inert, you don't need to flush it out.