Oh a bit of similar situation I was in... 3 months ago I brought home a male puppy to our 2 year old male, like you I thought it would be no problem because he always played with other dogs well. But, no, not when a new dog is invading their territory
I am going to say give it time, for us this is what worked -
play with them together in a neutral zone, when we give them treats I always make sure the oldest one gets it first, I feed them both at the same time, but in different spots, for me this was the only way because the puppy is a bit of starving marvin type and will jump dishes to eat others food. Take them for walks together is great, because of our winter here I was not able to do that with mine, but it is a great bonding exercise, anywhere you read about introducing a new pup it will tell you this.
Also make sure you give your 4 year old his space, for me, we had a special "quiet time" spot for him in the couch where the puppy couldn't reach him, because he would get so stressed that he would hide and not come out. So you really have to do baby steps and take it slow, don't push them into being together and getting along all the time, just a bit at a time.
Its been 3 months since we got our puppy, and now they get along, but I can tell you I still don't leave them together alone during the day, number 1 because the puppy is still house-training, but I also feel they are not ready to be in the same room all day long, I am afraid I would come home to someone being hurt, as even when I am watching them play I have to keep reminding them to be gentle - and then someone ends up crying.
I wish I could tell you I have a magic solution for yah, but I am still working on my situation, but it does take time, don't get discouraged so soon, I remember reading somewhere that it usually takes 3-6 months for dogs to bond. And even if they don't become the best friends ever, they can learn to live together, it just takes time, training and patience, lots of patience. I was contemplating re-homing the puppy after a few weeks, and now I am so glad I didn't.
Now, I am not saying is the same situation with everyone, but that was my experience. I hope it helps. And I hope you can upload pics of your furbabies soon
Your pups have both very strong personality, so I would assume its going to take work, my brother had a golden lab and a mastiff that practically grew up together, just not in the same house, once they were living together they fought allot, because one was invading the other girls house, with time it changed but it did took a bit of time too.
I hope Tenderfoot will give you some advice
she is very smart and full of very good advice!