I know, it does make it difficult. I used to take my levo at night which helped a lot! In that case, if I needed to take iron 3 times a day, I would take it right after breakfast, after lunch and then with dinner if it was around 6 ish. I would then take my levo right before bed, 10ish.
Of course, not everyone gets on well with taking their levo at night but taking it in the morning can make it a little more tricky.
If you take your levo in the morning you could then have your iron with lunch, dinner and then with a snack later in the evening.
It can be difficult trying to take everything away from each other, but 4 hours is plenty and it should be at least 2 hours away, but 6 hours is probably overkill unless it specified that it should be 4-6 hours.
I hope that helps