That's normal for 4 months old.
A change happens after the "fourth trimester" and sleep changes from being as it was in the womb to being more adult like.
In the womb and through the fourth trimester (up to about 3.5-4 months old) baby's sleep is passive. This means sleep is the normal state - as long as all basic needs are met (nourishment, comfort, safety) then baby will be asleep. It's a passive thing.
They come out of that phase at around 4 months. It's often called "the 4 month sleep regression". It's not a regression though. It's progression. It's a permanent change, baby will never go back to the passive sleep of a newborn.
Post 4 months, sleep becomes an active endeavour. Sleep develops onto cycles with periods of lighter and deeper sleep often lasting in cycles of around 40 minutes.
The process of going to sleep is no longer passive, it is active and needs effort to go to sleep. Not passive like it was. Staying asleep is also active. Now sleep is in cycles baby needs to develop skills to be able to go through from a light sleep phase into a second sleep cycle without waking up.
This is why you get short naps starting at around 4 months old. It's nothing whatsoever to do with the dummy. If you used a dummy or didn't makes no difference - your baby would still go through this physiological change at 4 months old.