Hi Sarah,
Great question! Try at the first nap of the day, laying him down quiet yet awake and see if falls asleep on his own, from the awake state. Keep practicing as this becomes more successful...at different times in the day. If you don't like crying or sleep training. This is the number one goal for you and your family to focus on for the next 8 weeks.
You can try at nap #1 of the day - quiet, yet awake. give it some time (5-10 mintues), see if he settles himself to sleep. If you lay him down overtired, he will be crying and that is normal...give him a few minutes to see if he can self soothe to sleep.
For the 4th trimester you want to focus on soothing to sleep, generally. And offering opportunity to learn how to fall asleep on his own, as listed above.

Here are some additional resources:
1. http://www.mybabysleepguide.com/2013/02/average-sleep-charts-by-age.html
2. DVD - teaches soothing and swaddle. happiest baby on the block by Dr. Karp
3. DVD Dunstan Baby language. helps you "hear" what your baby is telling you.

Purple crying period
http://purplecrying.info/what-is-the-period-of-purple-crying.php

Blog posts:
Sleep Training Methods
http://www.weebeedreaming.com/my-blog/sleep-coaching-methods

4 month regression
https://www.sleepbabylove.com/4-month-sleep-regression-suck/

Teething:
http://www.slate.com/articles/life/the_kids/2015/05/teething_symptoms_in_babies_are_not_as_bad_as_parents_think.html

Hi Christian,
This is a typical time when the brain advances and we need to change our nap and bedtime soothing method. Laying them down in a quiet awake state is important. Try at the first nap of the day to do this. Increase the skill as it gets stronger to other naps.
http://www.mybabysleepguide.com/2013/02/average-sleep-charts-by-age.html
Offer naps at the age appropriate awake times. See chart.
Learn more about the 4 month sleep regression here: https://www.sleepbabylove.com/4-month-sleep-regression-suck/
A great way to help your LO, in addition to the quiet yet awake nap/bedtime routine. is to sleep train. Her are some methods you could use: http://www.weebeedreaming.com/my-blog/sleep-coaching-methods

Hi Shealyn,
Lets look at her schedule. Can you tell me more about her day? She sounds very overtired. When you offer sleep at the right times, they can become more rested.
Try to follow the info on this chart for offering naps: http://www.mybabysleepguide.com/2013/02/average-sleep-charts-by-age.html
Have you considered sleep training? It is learning how to soothe to sleep without help from parents. The main skill is falling asleep from a quiet, yet awake state.
Then responding to her in the same way everytime she wakes up. This will show her what you want her to do.
http://www.weebeedreaming.com/my-blog/sleep-coaching-methods

Well she normally wakes at 6 and her morning nap usually happen between 830/9 and only last 30 minuets. Then she nurses at 1145 and goes down for a nap. For a while again it was like 30-45 minuets but then it went to 2-2 1/2 hours, but we have gone back to the 45 minuets again! I try to rock her or get her to sooth herself but it doesn’t work. And when she does fall back asleep I put her down and she’s back up!
Night time she goes to bed at 7. Most nights she then wakes at 10, then 12:30 then 3/4. The 1230 one is easy to get her back down as I usually nurse her but the other two can take up to an hour for her to go back to sleep.
It’s hard for me to do the cry it out method because my husband and son are sleeping

Shealyn,
"it is hard for me to do the cry it out method" Try to remember that crying is part of learning how to self-soothe. With help and guidance, the crying is timed limited to 2 weeks. Also, remember, that an overtired baby crying too. There is much crying and fussing for an overtired baby. Being overtired is not maximizing their growth and development.
A couple more tips: They must fall asleep independently to sleep well during the night. If they were rocked, nursed, sucking to sleep with a parents help, they will need that support at night too. If they self soothe, they will learn to self soothe in the middle of the night too.
Please click on my photo, it will give you more information about what packages we offer to help you family have a custom plan to get everyone - back to sleep.
Maybe dad and son can go on a trip/ sleep at grandparents house/auntie's house so you have the time and space to teach this skill to your baby.