I don't think there is any amount of sex you should ever be having. I mean, it's totally dependent on the person. And after 4 yrs of being together, it wont be as much as you used to and actually it may even become less the more time you are together. By no means think it's personal You said you guys get together once a week so he must find you attractive to want to do it even if it is just once a week.

The most important thing here is to speak to him. Tell him how you feel