Spatz try and sort out the genuine advice you have received from what I like to call the 'bitter first wives club' (although the 'golden uterus' coined by a op is a line I am happy to steal) however I want to come at all of this from a completely different angle.
I want to talk about YOU in all this.
Here you are stuck in the middle of what sounds like some pretty toxic parenting from BOTH parents. I believe you have nothing but the best interests of this child at heart but also think you MAY be being a little naive.
There is a 4 year old boy. Your partner and his child's mother share care 50/50. (see how clever I am to work out what bio-mother means without being a rude arsy git ?) .
From the scant information we can glean from you tells us that your partner works until 6. Pm.
So without you - this child would be in (expensive) childcare at least 2 days a week after school. But handily - you can relieve your partner of this cost by collecting him. But that wouldn't be YOUR partner caring for him. It would be YOU caring for him. After all he is only 4 - so would be in bed not much more than an hour after your partner got home. The reality being that on the 2 or 3 work days your partner has 50/50 care he would be 'looking after' his child for less than 3 hrs a week.
Now compare that with the other parent . Who probably didn't want to share 50/50. She has to sacrifice half of all time with her child for that child to spend time with her ex
Partners girlfriend. I am sure if this were YOUR child - you would not be happy about it at all. Sharing your child especially when very small with another parent is hard enough. Sharing with an unrelated person would be unbearable.
In my opinion having children requires sacrifice. I am a mother and stepmother to 8 in total. I can tell you this for nothing. I am not a mug. If MY DH decided he wanted 50/50 then HE WOULD BE DOING IT !! Not me.
I would not sacrifice MY earnings, MY wages, MY future pension - which I would have to do to fit in with school pick ups etc. No way! I adapted my working life for MY children. DHs children are HIS responsibility. !
This does not mean that I don't love and care for them . (All late teens now) but the sacrifices required to care for other peoples childcare responsibilities are not mine to make.
Therefore DH didn't have 50/50 until they could get about unaided. He talked the talk but in reality could not walk the walk by cutting his hours (and income) to make this work.
So on to you OP. Why are you free at 3pm everyday to facilitate your partners pretence of being a 50/50 carer. Perhaps you have an unusual but steady shift of work between 7-3pm ? Possible but quite unusual in a safeguarding role ?
If I am right - and I fear I am, you are either working part time or cutting your hours of work, and all the associated benefits of being in (or looking for) regular full time employment in order to support a lifestyle choice that your partner has committed to without ummm 🤔 much commitment.. and without a wedding ring on your finger - the only loser in all of this will be you.
I understand his exes anger .
I truly believe you are a kind person .
I think your partner is either thoughtless or manipulative or perhaps both.
Either way . Invest more in your future.
50/50 means just that. He needs to do it. Not sub-contract 40% of his work to you !