Let him sleep where he sleeps. If he falls asleep on the couch, you actually disrupt that by picking him up and taking him to his room. Is there a pattern to when he falls asleep on the couch? If he's falling asleep at 3 p.m., that's a nap. If he's falling asleep on the couch at 7 p.m., then that's bedtime, and 8:30 is too late for him.
Take all that stuff out of his room. Put a dresser with drawer locks in there, and bolt it to the wall if it's more than 3 drawers high. If there's a bookshelf, bolt that to the wall too, or take it out. Take the toys out, and leave one or two of his most important stuffed animals, and that's it. Put a doorknob cover on the closet door and the inside of his bedroom door so he can't get out. Or, put in a Dutch door - a lot of people are going back to those for kids' rooms and even kitchens if they want to keep toddlers and pets contained. It allows light in so you don't need a nightlight, but the child can't get out. (If there's no stuff in the room, he can't use anything as a step to help him climb out.
What's the bedtime routine? For an hour before, there should be downtime. Quiet bath, story, lullaby, cuddle with stuffed animal. No screen time, no TV, no running around, no starting some fascinating project or game he won't want to stop. After we did bath and teeth, we read a book or two in his room, then he got in bed. Sitting next to him or lying next to him once he was in a twin bed, my husband or I used to sing "Good Night, David" to him to the tune of "Good Night, Ladies." Some of the verses were the same every night (like Mommy loves you, Daddy loves you, Nana loves you...) and sometimes I sang "We had a good day, we went to Nana's, we had some dinner, and then we went to bed." Just make the words fit based on what you did - "we went to the library, you played with Jimmy...." Anything, but put the unique activities earlier in the song so the routine verses are at the end. I sang softer and softer, slower and slower, and it helped him calm down. Same thing every night. Routine.
I don't have a strong feeling about melatonin either way. I know some people really need it - especially kids on the autism spectrum. But I also think people reach for a pill all the time just to make something happen immediately before they try other techniques. If you do use it, talk to the pediatrician about a good brand - don't just buy junk off the shelves that may not even have Melatonin in it! (Remember the big brouhaha a few years back about GNC and Whole Foods selling various supplements that didn't have the named item in the bottle?)
If you aren't going to bed until he's asleep, and your normal bedtime is at least 10 and maybe 11 p.m., then you're saying your kid is up from 8:30 to 11 p.m., and that says you have to change what you're doing.
Mine napped for 3 hours well past the age of 4, but if yours is sleeping for 4 hours, it's because he's overtired from not sleeping at night. Remember that deep, continuous sleep is essential for brain development in children. You've got to get a handle on this.