Some of this depends on the size of your daughter. Many four month olds aren't very big, and aren't quite ready to learn to soothe themselves to sleep. Until you feel comfortable letting her cry (when you feel that she and you are ready), then I recommend getting a sling. When I felt that my son still needed help falling asleep (3-4 months old), I put him facing me (upright) in a maya wrap. I could easily swing him around without exhausting myself or hurting my back! It's easy to loosen the ring once the baby is asleep, and gently place him/her back in the crib.
Once my son was physically big, and I felt he was ready, I decided to let him cry. It took only 3-4 days, though it was very very hard on me. I found, in reality, that he cried less when I WASN'T holding him before bed! He really screamed at first, but then quickly learned to soothe himself. Some say it's not good for babies (promotes feelings of abandonment, etc.), but I found that my son stayed extremely attached to me. I continued carrying him in a sling or baby bjorn throughout the day, and paid a ton of attention to him. He's a fantastic sleeper now, and has been since that time! Try the book Healthy Sleep Habits, Happy Child by Weissbluth. He gives variations in methods, so you can pick what works for you. Some kids need help learning to soothe. My son screamed louder if I came in and spoke to him or rubbed his back! You'll figure out what your daughter needs quickly.
Naps develop over time. I despaired of ever getting a break during the day, when my son was 4 months old! But once he started crawling (right before he turned 6 months), his schedule changed drastically. He started taking 3 naps a day, of about an hour each. Within a couple of months, two of those naps stretched to 2 hours or more! Crawling and playing makes babies TIRED. :) Also, put her down before you think she's sleepy. Most babies can only stay up for 1.5-2 hours at a time during the day, and then it's time for a nap!