I had the similar experience today. I bought upgrades for two 16 gb rams, definitely different brand from what's original to my iMac, and it wouldn't start. So what I had installed were 4 gb original rams on slots 1 and 3, and added the 16s to slots 2 and 4, and pc would not boot. I could hear the machine work but nothing loads on the screen. I heard this could happen if your rams are not identical. What I did after is I took off all rams and added the 16s I got to slots 1 and 3, and viola! It worked perfectly. I think it's probably the ram channels wanting the same ram profile.
As for your case, I am sorry it's not working with all 4 identical rams. Maybe try using just slots 1 and 3 with identical rams? Also make sure they are placed correctly and push them all the way in.