I also sleep-trained when my daughter was 4 months old. Until then, she was a tough sleeper - was a colicky baby who had to be held, spent many a night sleeping in her bouncy chair (yes, overnight) before she transitioned to a vibrating pack-n-play and then to her crib. We used The Sleepeasy Solution, which I thought was very easy to follow. I also read Ferber, which confused me with all his examples and jumping around with regards to kids' ages and situations. I didn't like Weisbluth's tone in his book, and Babywise drove me crazy. The methods in the Sleepeasy book were also more or less verified by a child development specialist I consulted with when I started sleep training (yes, I was SERIOUS about training!) At her advice, I started at naptime, set a timer to go off after 10 minutes, went in to pat her for less than 1 minute at that interval, reset the clock and so on. It was really hard to listen to the crying (and to look at the frowny, "poor thing" face my mother-in-law made) but surprisingly it only took two interventions that first time, then one the next. After a very short amount of time, she fell asleep without a problem and was a GREAT sleeper (until we took away her pacifier...but that was a couple of years later!)
Good luck to you!