I am little confused by your question. The depo provera shot is given once every three months not for three months at some unspecified interval. If you mean that you received the depo shot 4 months ago, and hence, the last month, you did not get a repeat depo shot, then the answer to your question is, yes, you can get pregnant.
I was on the depo shot for about 5 years before stopping to try to conceive. It took me 3 years to conceive my son who is 6 years old. So being on depo provera over the long term can cause the reproductive system shock when the medication is stopped such that it can be very hard for some women to get pregnant. I am back on depo provera and have been on it for another 5 years...total of 10 years. I don't plan to have any more children but I still plan in the future to get off of it because it causes severe bone loss. I take my calcium supplement 2x a day along with a multi-vitamin and my doctors have said, that slows the bone loss but doesn't stop it. I just haven't gotten up the courage to make the switch to Mirena (an IUD) but probably within the next 1-2 years. So also make sure you are taking your calcium supplements while on depo provera. You don't want to have frail bones when you get older and break a leg just by going down the stairs.
If you took the shot and waited till 4months to get the shot again then Yes it is possible BUT, big but, they would have given you a test (HCG test) and in therory it would have told them if you were....then they should not have given you another shot.
BUT it would be good to get a test and just check. Although depo plays around with your hormones a LOT...I lost my hair with my first shot and gain 60lbs in a month and half....so I HATE DEPO.
Get checked out to be sure....and even with Depo you should be using condoms and spemicide(One bc method for you and him and one for insurance) Plus you have to watch out for STDs....that bad stuff.