Why Short Naps Happen
Inability to Handle Partial Wakings
Your child is waking up after reaching the light sleep portion of a sleep cycle. If your 4 month old learns to fall asleep independently through some form of sleep training - the level of crying sleep training needs to involve can be up to your family - then it may solve this problem. The drive to sleep is much less during the day so it is more difficult for infants who do not fall asleep on their own to make it through a sleep cycle transition. These infants have a partial waking and realize the conditions under which they fell asleep (nursing to sleep, bottle to sleep, pacifier, being held, whatever it is) are not present and then they fully wake up. Richard Ferber's Solve Your Child's Sleep Problems has an excellent description of this issue. So one thing that will help consistently fix a short nap without parental intervention is fostering independent sleep at the start of the nap.
Another reason a short nap might happen is that your child can get stuck in an overtired/undertired loop. You indicate that your child is awake for 1.5 hours between naps and taking 4-5 naps a day. This is most likely an age-inappropriate schedule for a 4 month old, as I learned the hard way. You need to be moving to a solid 3 naps a day.
What is quite likely happening right now is that your child is not awake long enough to get quite tired enough to make it through the sleep cycle transition. In turn your child has a short nap and is overtired from not getting the full benefit of a longer nap, so your child exhibits drowsy signs earlier than a child who is well-rested. It can become something of a trap.
Since your child sleeps an age-appropriate long stretch at night your child is probably capable of more time awake during the day. You might want to look at sticking with 1.5 hours awake before the first nap, since babies often like the shortest awake period in the morning, but then stretch the awake time by 15 minutes or so more between each nap. So you'd do something like 1.75 hours before nap #2, 2 hours before nap #3, and even up to 2.25 before bedtime, although your child may need slightly less. I would gradually begin stretching time awake to see if it lengthens naps.
This advice is specific to the schedule mentioned above. For many children short naps are a sign of being kept up too long before naps causing overtiredness, but the above schedule is at least on the border of being too short.
Not Enough Sleep Left For Naps
Young babies can't handle very long awake at a time and therefore need to take a few naps. However if a young baby is well-rested at night there may not be a whole lot of sleep left for long daytime naps. Older babies who can stay awake longer during the day and therefore have fewer naps have more consolidated nap sleep, which tends to mean longer naps.
This website has a whole long list of other ideas, such as too much or too little stimulation, environmental factors, hunger, and so on for early wake-ups. In general a good search for "45 minute nap intruder" will get you a ton of hits on this issue with varying possible causes. None of the things I could find online like stimulation issues helped my infant, but yours may be helped by them.
Additional Possible Courses of Action
There are several other things besides fostering independent sleep you can try. First, try to conduct a nap extension. A nap extension is using any means that you are comfortable with that works for your baby to lengthen the nap. Possible options are:
- Allow some crying - Happy Sleep Habits, Healthy Child recommendeds up to 20 minutes at this age, I believe - to see if the child will return to sleep independently. This is obviously not a good fit for all parenting styles.
- If the child uses a pacifier to fall asleep, be in place just before the end of the sleep cycle and replace it just as your child stirs. Or be in place to recreate, in general, however your child falls asleep if it is not independently just at the end of that sleep cycle. This is the method recommended in The No-Cry Nap Solution. It worked all of once for my child.
- If the child falls asleep nursing or with a bottle, when the child wakes be prepared to feed back to sleep. Keep the child with you for the duration of light sleep (15-20 minutes) before returning to a sleeping location.
- Since you used to hold your child for naps, hold until asleep again after a wake-up plus the duration of light sleep, then return to the child's sleeping location. This consistently worked for my short napper who was also held to sleep.
- You can also try placing a "heavy hand" on your child through the full light sleep cycle transition of 20 minutes. Be in place 5 minutes before your child usually wakes up (25 minutes for your sometimes-30-minute napper) and place your hand on your child's chest in a reassuringly firm manner. Keep your hand there as your child stirs throughout light sleep until the child returns to deep sleep.
- If your 4 month old often starts awake by kicking or jerking, you can keep your child from jerking though the light sleep cycle. For example my infant used to kick herself awake. I would try, and occasionally succeed, to keep my hand over her legs to muffle her kicks. This kept her from waking up during light sleep. A product like the Baby Merlin's Magic Sleep Suit purports to do the same thing. This method is mentioned in Secrets of the Baby Whisperer.
By conducting a nap extension you may help teach your child to learn to sleep longer for naps and you may find naps naturally lengthen on their own.
Some parents have luck with another method from Secrets of the Baby Whisperer known as wake-to-sleep. You enter your child's room at 25 minutes, or 5 minutes before the usual wake-up time. You gently stroke your child's cheek or do the minimum to get your child to lightly stir. The book claims that this restarts your child's sleep cycle. Over time it may teach your child to make the sleep cycle transition on his or her own, especially if your child learns to fall asleep independently.
Sometimes None of This Works
The author of Happy Sleep Habits, Healthy Child, Dr. Marc Weissbluth, mentions that between 4 and 6 months of age some babies don't nap well during the day. Many parents who have been in your place will agree. However the good news is that nap length evens out as babies age. Most short nappers begin to nap longer once they take 2 naps consistently, which appears to happen between 5 and 8 months of age. For some babies the transition to 2 naps occurs because naps naturally lengthen. For others the time awake begins to lengthen. For yet others night sleep becomes disturbed by extra wake-ups or an early wake-up in the morning indicating the time for a schedule change has come. However it occurs, when the transition to 2 naps occurs many formerly short nappers begin napping at least an hour for each nap. For a few babies naps do not lengthen until the transition to 1 nap and some babies simply only need a 30-45 minute nap and will never have long naps.