Agree with pp that you have to teach this. Just keep practicing and they will get used to it.
Get out a “special” box of quiet time toys which only come out for this purpose and sit him on the “quiet time” rug and tell him that this is when mummy needs him to be quiet and he needs to play by himself for a while. Those clocks with the bunny ears help. Start with 10 mins and work up to half an hour. And start training the little one the same.
Start after lunch at a weekend or while you are prepping dinner. Follow through with it and don’t respond to chatter. Just lead them back to the mat and say “quiet time”,
You can reinforce it with humour by demonstrating a closed mouth in an exaggerated way, a bit like you would for listening ears. A four year old should be able to tolerate thirty minutes or so.
Is there anything exacerbating this? Does he get enough one on one time with you during the day? Is he jealous of all the attention being given to the little one (in his eyes)? Maybe good to schedule in 20 mins one on one time with him when his sibling is asleep?
And name it as “Mummy’s special time with “Jake”.”
Remember to stress all the positive aspects of being older: “Jake gets to stay up with mummy after the baby has gone to bed”
”Jake is so big and strong now he can help Mummy look after the baby”
“Jake gets to sleep in a special bed and eat this yummy food with us as he is older”
Etc
Does he get enough exercise and sleep? Does he eat well? Are any of those things contributing to his neediness? Are you getting enough sleep? Are you getting enough support from the dcs’s father? Can you buy yourself a bit of peace and employ a baby-sitter for a weekend morning?
Also try and explain to him how time consuming (in a funny way) looking after a little baby is and suggest that he helps you?
Once you have put the above strategies in place, try and relax and acknowledge that you are doing the best you can, You are deep in the trenches currently op. It’s really hard. 💐
I wouldn’t normally recommend this for parents of young children who need to keep their wits about them, but in extreme circumstances, if your mh is suffering from the noise, there is a brand of ear buds called something like Calms ???? or something - have a Google - which allow you to hear but cut out the top notes of whining or shrieking. Obviously you have to use them with care, never at bath-time for example, as kids are designed to cry and shriek to keep them safe. Good luck!