My son is 4 months old - is it too late to introduce sleep suit? Is there much benefit at this age if they haven’t used one before?

Using a swaddle/sleep suit, like the one you describe, is the same as wrapping in terms of safe sleeping.

There has been some conflicting in evidence recent years in relation to wrapping/swaddling babies who are not used to being wrapped/swaddled (including using swaddle suits)
Some researchers have concluded that wrapping an infant naïve to wrapping may increase their risk of SUDI….......concerns have been raised relating to infants who are not used to being wrapped
This finding has led to a recommendation by some authors that ideally, infants should be wrapped from birth, if parents choose to use this infant care strategy
https://rednose.org.au/article/wrapping-babies

Some babies do not respond (settle) well to wrapping & may resist. These babies are generally better not wrapped.
Also, babies who are rolling should not be wrapped & the general recommendation is to modify wrapping from about 3 months of age in preparation for rolling.
If you wrap your baby, consider baby’s stage of development. Leave arms free once the startle reflex disappears around 3 months. Most babies eventually resist being wrapped. Wrapping style should be appropriate for the baby’s developmental stage.
It is essential to discontinue wrapping as soon as baby starts showing signs that they can begin to roll, usually between 4-6 months of age but sometimes younger
https://rednose.org.au/article/is-it-safe-to-wrap-swaddle-my-baby

In light of the information above, it is safer to not re-introduce wrapping/swaddling after not having used it for a while.
Many babies are also starting to roll from 3 months of age.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over