If you want the best sound for the 4.0 is the origingal 40 by Flowmaster. Alot of people here use them including myself, but I have a 3.0 and it sounds really good. I owned for a short time and then sold to a friend, in need of a truck a 94 4.0, 5spd ranger and It had 60,000 miles on it when I bought it, used to be a forestry dept truck for CA. I got it and did engine maintence on it and then I changed the whole system from the catback with 2.5" pipe to rear of the tire and installed a original 40 & with a 5spd and healthy engine that 4.0 was strong, fast, the sound from the OG 40 was amazing. I drove it for sometime then my friend bought it from me.
He then put a K&N intake with a cone filter and that 4L even sounded WAY better. I was still able to drive it here and there and heard it from a distance alot and I was happy to hear out that sounded. I only put on a SI/SO 40.
Just remember no V6 will ever sound like a V8 no matter what is done. If you want duals use 2.5" SI/DO OG 40 and use a good pair of 3" tips. Some swear that the super 44 and super 40 sound better, all in all I have heard alot of 4L with different types of exhaust setups after having and working a 4L myself I swear the OG will sound the best on your 4.0.
It's all in suggestions and I am stating mine. I would use the OG 40, but it's your choice. One more thing don't ever put glasspacks on your Ranger it sounds awful, glasspacks don't even make a V8 sound like a V8, it sounds more like a V8 ricer with glasspacks.