Unfortunately this is very common once a bub turns 4 months old. My DS2 went from being a great sleeper and settler to the absolute opposite when he turned 4 months!

Keep persevering with what you are doing, it will pay off eventually. Re: the napping for one sleep cycle, I'll tell you a trick that worked well for us. My DS2 has 40 minute sleep cycles, he would wake up after one cycle and refuse to go back to sleep. I started clock watching and I would sneak into his room a few minutes before his 40 minutes was up and watch over him. As soon as he stirred slightly (he would squirm, sigh and move his head) I started patting him gently on his tummy. This would help him go into his next sleep cycle without waking. It didn't work EVERY time, but it would work for at least one nap a day. Eventually he started working out how to go back to sleep by himself (when he was around 6 months old).

Best of luck