rakaats

Doubts with regards to the number of rakaats performed

Q: In our local shopping centre there is a small prayer room. Three of us entered to pray zuhr salah and decided to pray in jammat. I was chosen to lead the prayer. We finished salah and went our separate ways. My question is this, after salah and returning home I kept feeling I prayed only 3 rakaats. I just couldn't get this feeling out of my mind. When we finished the other two never mentioned I only completed 3 rakat, so to tell you the truth I do not know whether I completed 3 or 4 rakaats. What must I do or should have done?