It's really normal to have short naps until 4 to 6 months (sometimes longer) so I would focus on the wake windows more than the length. You can work on length later. And it's okay if you rock, cuddle, or feed him to sleep. :)
At 4 months, a typical wake window is 75-105 minutes. He may be ready for his first nap as early as 1 hour after he wakes for the day. I would look more around the 60-90 minute mark and watch for the common sleep signs. It's the best way to learn the wake window. Just spend a week doing that. It truly can make a huge difference. I've done a whole 40-minute video on it but that's where I would start.
For many, when you nail the wake window, they can fall asleep easily and sometimes just going beyond the wake window by 5 minutes is enough to create a battle. It's an important first step before even contemplating sleep training. With sleep, the most important thing is to find out the cause of the sleep challenge and know what's age-appropriate. Once you understand the cause and address the cause, (overtired, reflux, overstimulated, sensory,... the list is endless), the solution is much easier.