Trust in a relationship is huge portion of the relationship and when you find out that the trust you had in another is blown out of the water it's very hard to get it back again.
That being said, I think we have to also understand the reason behind why someone might lie to us. Maybe your boyfriend really wanted you to feel very special and he told a small lie for the moment of making you feel good. Then 4 months goes by and the lie is eating at him and he possibly feels he would rather have a truthful relationship and come clean with you about the lie.
I don't think we "just get over it". There is a process of forgiveness we go through (if we forgive at all). You have to think about it, feel it, decide for yourself if his intent when telling the lie is acceptable to you or not and if his intent in coming clean with the lie was in the interest of your relationship together etc etc.
Give yourself the time to think about it and process it. Communicate with him about it. Maybe through the past 4 months of your relationship he has learned how much truth means to you....and he wanted to be truthful.....there could be many reasons.
Wishing you both well!

sabby