I think what GoodWood said was fairly accurate. I will try to expand a bit. However, I'm not really sure how accurate I'll be so bear that in mind.
I don't actually know which individual drug came first but I don't think it was the alprostadil. I believe Prostaglandin E1 (PGE-1, Alprostadil) was the first drug approved (only?) for that use by the FDA. That approval was for the proprietary alprostadil mono-mix preparations of Edex and Caverject. I don't think any of the compounded mixes ever got specific approval, including the compounded mono-mix Edex substitute I use.
The alprostadil is generally considered the heavy hitter ingredient of tri-mix or quad-mix and is usually considered to be the least likely to cause damage although some later research indicates alprostadil may not be as benign as originally thought. Unfortunately, as it happens, while possibly the least likely to cause damage, alprostadil is the most likely to cause pain or soreness. Although that's not an issue with many men and the pain diminishes or goes away in other men with continued use (I'm one of those).
All of the drugs work a bit differently and there are several reasons for using bi-mix, tri-mix or quad-mix. Some men can't tolerate alprostadil at all. The bi-mix eliminates that ingredient. Some men find that by adding the other ingredients of tri-mix they can reduce the alprostadil to a level they can tolerate. Some men don't get satisfactory results when using alprostadil alone and find that adding the other ingredients gives them the desired results.
The ratios of a mix can be played with to fine tune the results for the individual. Quad mix can be an even stronger concoction for those that are no longer getting satisfaction with the other mixes. I believe the combination of the drugs in the combination mixes are thought to work synergistically making the combination more potent than just the added effects of the three. I believe the quad-mix can also be a concoction that adds a pain killer that allows the use of alpostadil for those that can't otherwise tolerate it.
It seems that many of the compounding pharmacies that sell injection drugs for ED don't offer a mono-mix and only sell a bi-mix, tri-mix or quad-mix, or possibly only a tri-mix. It also appears that some, possibly many, only sell certain pre-set mixes and don't do custom mixes to the doctors order. The prescribing doctor has to pick a mix from a list. I believe some pharmacies get pre-mixes that they just add saline or bacteriostatic water to. I don't really understand the reasons for all of that.
I'm still using an alprostadil mono-mix as I've learned to work around and tolerate any pain or discomfort of alprostadil, it may be the least likely to cause damage, it works well for me, and I figure why add more drugs than necessary. If it didn't work for me I'd use whatever worked. If my situation changes I'll try something else.
Wedgewood Pharmacy, a pharmacy I have no experience with, has a pamphlet as a PDF document that explains the mix ingredients fairly well which can be found here:
http://www.wedgewoodpharmacy.com/uploads/ED%20Switching%20Guide.pdfBear in mind that I suspect I've presented some inaccurate information. So salt is in order. If someone would be so kind to correct any inaccuracies I'll correct this post.