Hey sorry this got missed, and (((hugs))) I think everyone struggles with 4mo
Did you miss out an 's' from 'yesterday's routine (ok I realise it's no long yesterday but you know what I mean?) or did he just have a really long first A time that day? Do you have a consistent routine with DS1, ie nursery drop off/collection the same time every day? And if so, what times are you out for this? Tbh with my DD (second LO) I ended up giving her lots of naps in the sling due to preschool drop off and collection for DS, it really helped to cover her face a bit (ensuring she could breathe!) so she wasn't distracted by everything around her. For both of them I found it helped a lot to get at least one good sling nap each day, as they both tended towards short naps - well DS really was a chronic short napper
But at least I could resettle in the sling and kind of reset them a bit, so they could manage a longer A time after their sling nap.
On the other hand, sorry if you mentioned this before, but can he get himself to sleep independently? If not, you may be running into the situation where he needs your help to transition between sleep cycles, because that's how he got to sleep in the first place. Even if he can get to sleep independently, it seems many LOs don't learn how to transition themselves into the next sleep cycle until a little later on, but either way, he should get there in the end
I do tend to agree with Martini in that he's likely to be OT from the short naps, but he does seem to be getting a nice long one in the afternoon, so at least you know he can do it
It is hArd when you have an older LO though, so much is out of your control, but it does get easier
If you could perhaps post your timings with DS1, maybe we can come up with a plan of action?