My baby used to take no nap at all during daytime, and all the 4 different pediatricians i went to see did not give me practical tips nor solution. At the end I went to see a sleep specialist and finally cracked the problem.
What frogprincess said is true. To start with, establish a nap routine and decide where you want the baby to sleep ( e.g. cot). A nap routine should be something easy and not too long ( probably 10-15 mins). Mine is close curtain, then sleep sack, cuddle, then put her to bed when she is awake and sing song for a while. Since she is still young, you could give her a bit more help like pat pat but eventually you need to reduce it to none. Also a darker room make nap easier, but no need pitch dark. Have sth white noise like fan to mask out other noise if your baby is sensitive to sound.
Apart from that, you need to know your baby awake time ( start counting after she wake up). At her age, it probably 90min to 2 hour, each baby is different, also watch the sleeping cues. If it is too late, she will be overtired and it will be hard to settle her, and he/she will likely end up crying. If it is too earlier and not overtired, they are likely to play with themselves. So play around +/-15 mins if the baby does not fall asleep within 15 mins.
Also you need to very consistent on the routine and try not to go out too often initially during daytime. Initially it was very stressful for me and my maid to get my baby to nap, my maid nearly cried doing it everyday. Now we could get my baby nap easily, and she usually sleeps more than 1 hour until i need to cut down her nap. Generally she is happy and alert nowadays
Pls do not use prop like bouncer and rocking if you do not intend to do this in the future, these will create sleep association and create more problem down the road and at one point you have to do sleep training to break that habit ( which mean let baby cry). 4 months is the best time to start before the habit get entrenched. You can pm me for more detail. I went through a lot of trial and error about baby napping issue and wish to share with other new mothers who face similar problem like me. Unfortunately Our mother generation only know sarung so usually cant help much.