The more concussions you have, the increased chances you have of getting another one. How far apart were the 4 concussions?
MissParker gave some good info for you. The fact that you lose consciousness and don't know what happened definitely shows signs of a significant concussion. You really should be evaluated after each concussion and then put through tests to confirm that you are ready to return to play. The risks for second impact syndrome also increase after each concussion is received and especially if the person returns too quickly.
For things that you can do - I have seen some padded headwear, but I know that many scoff at them due to them being for "wussies". Another thing is starting to take creatine daily. No need for excessive amounts, just the 5g per day will do well. There are some preliminary studies that have shown creatine having a great benefit to brain activity and recovery and some are hypothesizing that it may help brain recovery after traumatic injury. I do not believe there has been any concrete research evidence to prove it (though I have gone through the literature recently), but in the end it is definitely NOT going to hurt you (unless you have some underlying medical condition) to take the 5g creatine daily, as long as you take a high quality brand that doesn't have tons of fillers in it.
From the sounds of the mechanism of your injury, it doesn't seem to be a technical issue, but it might be worth just doubt checking to make sure you aren't lowering the head too much. Also, I second the mouthguard suggestion.