How long can you use a co-sleeper bed?

[KrisM]

My son is 4 months old. Until he was 3 months old, he slept in a craddle next to me in our room. He currently sleeps in his crib, in his room. He does fairly well there until about 2:30 in the morning. Then, he wants to be awake or with us. Last night, I slept on the floor in his room and he slept for a few hour without too much trouble. But, the floor is really not comfortable!

We've tried co-sleeping, but my husband moves around a lot and is a deep sleeper, and is worried about having him in the bed. Also, I then tend to sleep with my arm around him, which is making my shoulder incredibly sore.

I'm thinking the side-car arrangement might work. My son could be right there, and he could hold my hand while he slept. My husband and I would sleep better too. But, he's already 17 lbs and 26" long. Any idea how long we would be able to use this?

Thanks,
Kris