4 Months Regression vs Sleep Training

Hi,

I'm a 1st time mum with an active 15 weeks old baby boy. I've noticed a change in my baby's sleeping pattern and I am not sure if it's because he is approaching his 4 months sleeping regression or that he requires sleep training. 

A bit of background information. My baby is both breastfed/formula fed. Nap times are about 40mins - 1.5 hours and I will try to aim every 1.5-2 hours or whenever he is showing signs of tiredness (combinations of being nursed to sleep or carried in a sling).

He hasn't slept well since his birth and he only started to show some improvement around 9 weeks where he would sleep for example 830pm-1230am, then wake 3am, then 5am and then 630am. I would usually nurse him back to sleep with top ups at 1230am with his night feed. Around 12-13 weeks he slept 830pm-230am, then woke up at 530am and then 730am. He would take up to 3 bottles of formula, 1 bottle of express and breastfeed throughout the day.

I thought everything was improving until 14 weeks his sleeping is out of whack. Now he sleeps at 830pm-12.00am and seems to be waking up nearly every 40mins-1 hour till 6am where I've had enough and wakeup myself. On top of this, I noticed he isn't finishing his bottles as previously, however taking a liking to the breasts more (I don't have much milk supply hence why I do combination feeding). I've checked everything on the checklist from soiled nappies, burping, feeding etc - but it seems he is comfortable being nursed back to sleep (for about 5-10mins).

My diet hasn't changed, nor his bedtime routine so I am thinking is he approaching his 4 months regression. My question is - do I allow his to play it out like this for the next coming weeks (is this the norm) or should I introduce sleep training (CIO) method? I am concerned as my nipples are getting very tender and carrying/rocking him to sleep is out of the question as I have sever pelvis pain. Any similar experience or sleep consultants that can assist would be much appreciated. Thanks in advance.