It's hard having the confidence trying things with bub. But you can only try because what you are doing now isn't working for you anyway, so you actually don't have much to lose. I also think that it is best to ignore advice you are not okay with. I have found this hard as people very close to me have advocated crying it out and I am not comfortable with it, and now I think, "I am the mummy, I will do what I want/what I think is best for my baby".
I second (third) No Cry Sleep Solutions as a terrific gentle sleeping book. Perhaps you could try getting your DS to sleep in your bed during the day for naps with you, to start with. I know he likes movement but if you want the transition from sling to cot it has to be done slowly. In the bed, lie on your side and maybe breastfeed him to sleep. The first few times/days you can have a rest too! Then maybe try and sneak away when he is asleep. Don't make too many big changes at once. I think the trick with feeding to sleep is to make sure they are really asleep before you detach. I wait until my DD's mouth has pretty much slipped off before moving her. Too early and she screams! After maybe a week or two (or whatever) napping in you bed, you can try the cot with settling techniques that have been suggested above. Another key point is to make sure he is not overtired or else getting a bub to sleep takes a long time and then they don't sleep well at all ie wake up too soon, wake up grumpy etc. I think at 4 months their awake time is about 2 hours before tiredness sets in. this includes feeding etc. If it goes beyond this time, do anything to get him to sleep. Ngala are good for advice (08) 9368 9368 on sleep stuff (and other stuff). They may have some good hints on sling to cot transition. GL.