Re: 4 week old will only sleep on me!
Been there, done that. I just dealt with being majorly sleep-deprived. We had the Arm's Reach co-sleeper, too. I didn't co-sleep in the bed except for a few times out of total desperation.
I never knew it was possible to be that sleep-deprived and still be alive.
At that age, I held him for naps because he would wake up if put down and would not nap at all if I wasn't holding him. He did sleep in his co-sleeper at night, in a Halo sleepsack swadde with his arms swaddled down, although he would wake up very very frequently (but would fall asleep again after nursing).
All I can say is - what you are experiencing is normal and it does get better. It eventually passes and you will start to be able to get some sleep, although it will seem like that day will never come.
Proud mom to a perfect little boy born 3/16/11
and a precious baby boy born 4/24/14