i was at MK at rope drop last sunday (1/23) and since this was our first visit to MK, we had no clue where to go once we got in. I sensed other people did, and it took about 10 minutes to get to Seven dwarf mine train. It was my wife, our 4 year old daughter and myself. We marched through the line pretty fast, about 10 minute wait.
My 4 year old daughter is 44 inches tall (yes, she's quite tall) and she does well with roller coasters (ie. our local six flags). She went with my wife, since they only allow 2 per row. My wife said she was crying and balling throughout the whole ride. We have the evidence with photopass/memory maker. After that, we spent the whole day having fun and the next time we went to MK, she told me, "Dad, i wanna try it again". So even though i canceled our FP, we waited "60 minutes" (Disney lies, it was not even 20 min wait). She loved the ride the 2nd time.
As you know, 4 year olds are angels one day and demons the next or vice versa. My suggestion is to ease him/her into MK, maybe with Barnstormer or Tea cups and see how they react. If they react well, then try 7DMT (as long as it's not a crazy wait).
BTW, my daughter said the highlight of the vacation was going on 7DMT.