Oh @Paws4thought25 bless him! What I would suggest is, at least in the house, is give him a firm no and re-direct to his toy so he understands the boundaries - toy is fine, human arm is not. If possible always try and have a toy nearby. It means your house will be a bit messy for a few days/weeks until he gets the hang of being diverted but eventually (and if he's smart) when he gets very overexcited he'll learn to go to his toy. It's how I trained mine to handle over-stimulation and excitement. He goes to where we keep his 'sex toy' and waits patiently for it to be handed to him. Once he had a 'focus' for his outlet it meant that if I refused to give it to him he learned that he couldn't do what he wanted to do - so he just settled. There was a bit of 'grumpy' barking at first because he wanted it but he soon learned that he only got it sometimes. So inside, divert, make him learn that he can't just have at it whenever he wants, and teach him to settle.
Four months is a really rough age, as their brains are quite developed and they have so much energy that they don't know where to direct it (and obviously they can't be exercised too much) but they are also quite dim in terms of understanding social niceties and manners...
In terms of trying to mount other dogs, I would call him away from them before he notices them - get him to come to you and give him a v. high level reward. Lots of young dogs (male and female) hump in excitement but sometimes, the most social of them, will try and do it with other dogs. When they're this young there's no domination or nefariousness involved - it's just sheer stupidity and youth. The problem is, of course, that other dogs don't like it and it's v. poor manners. But really by 16 weeks he'll have done most of the socialisation he needs to do so there's no issue in recalling him from other dogs.
@VanGoSunflowers People love to criticise others when it comes to anything, puppy raising in particular (I have found). Ultimately, if a dog is healthy, happy, and reasonably well trained then you've successfully raised a puppy. Not all dogs need recall 100% of the time. Not all dogs need to stay on the floor. Not all dogs need to go to the groomers. And Pablo is a star, so ignore them 😀
@SpanielsGalore Quite! I imagine that particular mix will probably stay on the sofa for most of it's life. I can't see it working out in the fields. Bless P for being so terrified of toast though, what an odd duck! It's such a unique thing to be scared off. Poor P!