Unfortunately i didn't start BW until my LO was 4mos either and she took all naps previouly in her swing or pack in play, but mainly swing.
We did our wind down - Swaddle tight, carry dc around the house for few minutes, take to room, dim the lights, pull the blind (dc needs complete darkness), rock for a few minutes and then lie her down in bed. If she can't settle I would shh/pat until almost asleep. I knew i could leave she gave a great big sigh :-).
I hope this helps. Keep in mind that your baby will fight you. dc will also regress. both are normal - it will take time for him to learn the new way to sleep.