Thank you for the additional information, it helps
This age is a prime time for short naps so it is no surprise that he is taking 40 mins naps. You are doing brilliantly though to help him get that longer nap using W2S.
W2S is a method that is usually done for about 3 or 4 days and then you stop and observe to see if LO can transition alone, if not then you continue W2S for another 3 days to try to aid sleep training and instil the habit of sleeping longer. So it goes in cycles of 3 days then a day of observation. With some LOs going through the 4 month regression though it does seem like they will only transition with help each day so it ends up being more of a daily thing. It's can be quite tiring to keep doing it every day so I suggest you find a balance which suits you as best you can during this tricky phase. Some parents prefer to do the W2S so that LO has longer sleeps and is easier to deal with during A time, some prefer to accept the 40 min nap and get on with their day rather than trying to extend the short nap. it is really up to you. My suggestion is to try to keep things consistent each day.
There is another aspect of W2S though which is that if you shush/pat all the way to deep sleep (which the method suggests) at every transition time then LO might become reliant on the additional help and less able to self sooth. If you are shush/patting all the way to sleep at nap and BT and through every transition you might consider reducing this, keeping in mind the goal of LO self soothing and only giving the minimum help needed, always with the aim of reduction and weaning. I mention this as it can become habitual for parents (especially when very tired) to sit and pat and go into a bit of a daze do the patting continues rather than reduces. it can be hard to stay mindful to the aim of reduction when you are tired and when you know patting works, it is tempting to keep on doing it rather than risk LO waking fully. Just something to keep in mind.
He never seems tired enough for the catnap - he's been fighting it for more than a week now through in trying to enforce it as when I didn't he would wake not long after going to bed for night! Even if I only get 15 mins from him it seems to help but it's a battle!! Yesterday he was over 3 hours awake before the catnap in sling and he was happy! And tends to do fine even when been awake for this long?! On days he won't do catnap he could be awake 3 hours and fine?
At this point it is hard to tell if he is so awake because he's got a second wind from being OT or if he is low sleep needs and really is happy to go 3hrs.
If you find the sling works (or pushchair or car) for a CN later in the day then I'd just go for that tbh. 2 naps at home in his cot are good for setting good habits but this third nap being a struggle each day it is probably easier all round if you just go for the sling nap.
One of the reasons for sleep training and using the EASY routine is that babies get heavy - holding them for every nap is so easy when they are tiny and light but an older heavier baby is hard work - this third nap is the one that most babies drop at around 6 months so if it is APOPed now (sling nap) then this habit is not a long term one if you see what I mean, it is going in a couple of months even if he is regular sleep needs. Hope that makes sense.
since trying the DF I feel like now I've caused him to start waking now! And it's not even like the DF always buys us a good stretch of sleep - sometimes he wakes at midnight even after a DF! Though I haven't been feeeing him unless it's been 3 hours - should I stretch that to 4 since he's 4 hours in day?
You can try the DF a bit earlier to catch him in a different phase of his sleep cycle and try to prevent that 10pm NW. Or you can stop the DF if you find it is not helpful and just do a NF when needed. It is totally normal for babies this age to need feeds in the night and especially for a BF baby to increase night feeds to up your supply, this is the time of day your body gets the signals to produce more for his increasing need. Waiting until 3hrs is a good plan, there is not much point trying to resettle a hungry baby at the 3hr mark and trying to hold off to 4hrs in the middle of the night, much better if he's hungry to just get the feed done and everyone back to sleep asap.
I'm beginning to wonder if my milk supply is low and is that why he's waking? Though he would cry if he was hungry surely?
Babies who know they are going to be fed promptly because they are on a regular predictable routine and their needs are being met, may not cry for food. I didn't hear my DS's hunger cry until he was 10.5months old and that was following an illness and hospital stay after which he needed night feeds to make up for the calories he hadn't taken when he was ill. I wasn't expecting the need to feed at night so it was the only time he had need to use his hunger cry, which certainly got my attention! What I'm saying then is that on a predictable routine your DS may have no need to cry for hunger, however if you do not feed him and he needs feeding then yes he would let you know about it.
If you are uncertain of your supply please do post on the BF board where there are members with lots of experience and tips for increasing supply.
How can he be so happy in night just to lie there talking and babbling when he's had such little sleep? .
This can be caused by a routine which is not age appropriate but it can also as easily be part of the developmental process and the 4 months sleep regression. It is quite normal for babies to have phases of wakefulness in the night due to developmental leaps. Although it is worrying and tiring for you it is pretty normal for them, be reassured that he is happy. You do not have a totally inappropriate routine, quite the opposite you are offering a good regular EASY, and are doing your best to help him sleep and stay well rested which is all you can do.
This is a phase. As hard as it is, you will get through it. Let's keep an eye on the EASY logs and look out for any patterns which may emerge, OK?