私は四年間同じ会社に勤めています。
I think that would work too. At least that's what my sensei would tell me to use.
I don't know about this one either:
私は四月に千九百七十八年をうまれました
It seems for that one you should drop the "に" after "四月" and you should replace "を" with "に" since there isn't an indirect object. So it should look more like:
私は四月千九百七十八年に生まれました。
Though, someone may want to correct me on that.