I did a 4 box team recently when I only had 4/5 accounts active and wanted to blast out my weekly dungeons quickly, and I found that even with the tank being in the group queues are sometimes 10 minutes to find a healer. You'll definitely run into that problem when you level through 55-65 (because plenty of fresh DKs want to tank dungeons).

Tips if you choose to go that route:

Some people are going to dislike you, they are going to sit at the entrance and refuse to heal or make snotty comments. I find being nice is a good way to dissipate this reaction before it can happen by being honest and saying "Good morning, I'm just getting in some guild runs to level my bank guild, make sure you need on anything that you want for any reason." In 37 dungeons (just looked up that stat on my tank) I had 2 healers leave at the beginning, one without saying anything and one after saying something snotty. I also had a healer leave halfway through wailing caverns, although he didn't say anything so that could easily have been about wailing caverns being too damn long. I also had three or four healers that wanted to requeue specifically and several others that wanted to know more about multiboxing. I answered their questions politely as always. Talking is how you become a person and not a bot. That is always my social strategy when boxing.

Run passloot the addon and set it to pass any BOP blues your team can't use. I personally also make sure I manually tab over and pass any loot that the healer actually Needs on, needing on an upgrade and waiting two minutes while the other people in the instance let the timer tick down is the kind of thing that makes me irritated so I don't want to do it to others.

Make sure you have something set up to heal yourself and the healer if something goes wrong.

4 boxing is more stressful than 5 boxing because of that pesky other person, but not necessarily $15 more stressful.