we have made many attempts to put him down for his naps after rocking him to sleep.
The idea isn't to rock to sleep and then put down - the key and important thing you are teaching baby at the 4 month sleep regression is to go to sleep where they stay asleep. So not being got to sleep in your arms (or nearly asleep) and then put down, but going from fully awake to fully asleep in the one place, and not being moved.
The gentlest and kindest way to achieve this in a way that allows for independant sleep is to have naps in something that moves. I favour a bouncy chair but a pushchair (moved back and forth on the spot) works too if you prefer.
So you would put baby in the bouncer (or pram) when ready for a nap, start the rhythmic and non-stop movement at an even (heatbeat) tempo and keep going until baby is asleep. Then once asleep gradually slow down and stop. But stay nearby and any sign of movement from baby then restart the movement to resettle baby back into another sleep cycle.
It helps to also introduce an independant settling mechanism at the same time as this. A dummy is the best option for this IMO.
Longer term, you aim to use less and less movement until all you need do is put baby in bouncer/pram with dummy and no movement is needed to get off to sleep. That at point, you then move to naps on a stationary surface (ie the cot). Because by that point baby isn't needing the movement anymore, just the independant soother in the dummy.