"its like a power struggle and i end up giving up cos i’ve got the baby to deal with too."
You've just hit the nail on the head there.
I have a 5yo and she wants to be the boss of the house (her words) so always wants her own way, and I have a baby too. So I do get how you can end up giving up.
But that's the key, once they know they have that card up their sleeve, that you'll give up, they're the ones in control and they'll use all their tactics to get you to back down and to get their own way.
I think he's old enough to be sat down tonight and told: four-year-olds don't wear nappies. Five-year-olds certainly don't. We're stopping the nappies tonight. We've run out now and mummy and Daddy won't be buying anymore. Now my daughter would try and call my bluff so I'd empty the bag before hand and leave the empty bag in the usual spot. I'd then set out clearly, that if you don't wee in the toilet, you will be wet and cold all day. Mummy will not be buying more pull ups now. You have to wee in a toilet from when you wake up tomorrow.
Let him scream, let him cry, let him suffer the consequences over the weekend.
Let him realise there really is no other option, there is no going back to what is safe.
I will caveat that with, at the same time, I'd want to gently try and establish if anything is stopping him from wanting to try. Is he too scared of failing (e.g. has he had accidents and somebody has got cross and made a big huff about cleaning it up, because the baby is also screaming and dinner is on the hob, etc). I'd try and reset his expectations/allay his fear by saying it's OK if you have accidents as long as you're trying your best to go to the toilet.
Or is it literally an attention thing with the new baby? In that case, that's easy to solve by heaping on loads of positive attention elsewhere and trying to build in little moments of connection through the day. (Though it might be slow progress and not an overnight fix). My daughter has been craving my attention and I find myself saying no to games etc, so often. Either the baby wants me, or the baby would ruin what she wants to do, or I am just tired and can't be bothered, or got food to cook, or jobs to do.... So I have to be conscious of that because I am worried she's spent a year hearing "no, no, no, you don't matter!"
The dummy, I don't know. I think I'd have to "lose" it....