How often do you see your OB and get Ultrasounds?

Hi twin and multiple Moms, I am 22 weeks pregnant with B/G twins and am 38 yrs old. My pregnancy has been complication free and the babies are great. I am just wondering if I see my doc often enough? I have been seeing him and getting an US every 4 weeks. Do you think that is reasonable? I like my doctor. I am probably just a greedy worry wort! Your input would sure be appreciated!