Sadly, this is not unusual these days. There are a lot fewer ERs than there used to be and more people going to the ER for all sorts of things. My son, who is 4, waited for 4 to 5 hours to have his wound clued from running into the edge of a wall. They had to wake him up to do it and they didn't even move the hair all out of the way. Many who were obviously sick left and went to another ER.
You can absolutely write a letter to the hospital and you can get the names of those in charge and their addresses from their website. I wrote a letter to the same hospital after I had elective surgery that had been scheduled for a while and stayed in post-op for 6 hours because they couldn't find a bed for me. They eventually put me in with an old, blind woman in much pain after dinner so I missed that, too. When I asked one of the rent-a-nurses for more to drink, he asked me if I wanted to see where to get it so I could get it for myself! My roommate's daughter got it for me eventually.
After my letter, the nursing administrator called me to apologize and gave me her number to call if I had any other problem. She said her assistant had instructions to find her if I called. The floor nursing manager also called me to apologize.
If nothing else, the letter will make you feel better for having done something about it.