I know exactly how you feel. Both of my children have some speech issues, and it kills me inside to know that others judge them and tease them. My mama-bear comes out and I tell the other kids exactly what I am thinking. I don't hold back. I would want their mothers to intercede if my kids were picking on theirs. I think we need to be conscious, even at a early age, that people are different.
We are special and awesome because of those differences. We will see people that talk different, speech inpediments, cleft lip, foreign, downs syndrom, etc. We will see people that walk and run different from diseases or birth anomolies such as a shorter leg, curved bones, weak muscles, accidents, those using wheel chairs, etc. People's skin will look different from freckles, spotty pigment, skin disease, burns, cancer, etc. We are all different, and it doesn't matter what's on the outside as much as our personality and heart.
I think it would benefit your child and your peace-of-mind to have your child tested by the school system for speech delays. Many children have them and overcome them by about 2nd grade. It is common for young children to mispronounce multiple letters until they have a better understanding of how to read. I think the speech pattern you described sounds absolutely normal for a 4 year old. I have a 4 year old and a 5 year old. Boys generally develop speech a bit slower than girls, so he's not going to sound like an 8 year old yet. Testing through the school system is free.
You can also have your family doctor refer you to a speech therapist for testing. If you feel there are problems, I suggest a doctor test and figure out exactly what is wrong. The school system does not have very thorough testing parameters. What they consider as normal may be far below normal for what you-as a parent- consider acceptable and normal for your child. You know your child better than anyone else.
The school system tested my 5 year old and thought that his speech was just a bit behind and he was immature. I had him tested by a psychologist, and he said my son has Autism Spectrum Disorder. My son had several other issues in addition to the speech issues. However, it helped me direct the school system in knowing how to talk to my son, teach him, focus their teaching methods, and gave me a starting point to research ways to help my son at home and school.
I think that all children need help in understanding other people. We do not have automatic coping mechanisms to deal with differences, feelings and building social relationships. Even adults struggle with these things, so I cannot expect children to know how to act. We have to teach them. It takes a village to raise a child... I really believe that. Every person that comes in contact with our child is teaching them something. I try to make sure that the children around me are gaining a wise and knowledgeable insight on the situation instead of using their own judgment or whatever prejudice that has been taught to them.
Follow your instincts. They won't mislead you.
Fight on mama-bear! lol
As I was reading through the other advice you received, I just remembered that our children can be their own best advocate. I use those situations to talk to my kids about ways to cope and handle those things. Kids start calling names and often turn it into fights. I teach my boys to use their words. My 5 year old will tell any child... or adult for that matter... that his feelings are hurt, the person is being mean, that the person is making him angry, that he's not going to play with them because they are mean. I have seen other children start conversations with him and say sorry, change their attitudes, begin sharing, etc. Sometimes we don't know how our actions effect others, and I think it's important to learn how to voice ourselves. My son stands up for everyone. He's told his teacher, a man how is like 6'5" and 400 lbs, that he was being mean and he needed to be nice to the class. The teacher pulled him aside, wrote me a letter about it. I realized that the tone the teacher used was too harsh according to my son, and my little guy decided to let him know. And... you know what... I think it's important to keep that open line of communication. I think it helped the teacher to see how others view him, and he learned how to better address his class... and my son as well...lol.
I talk to my boys about situations on TV, cartoons, movies, people we see in public, books, etc. I ask them if the people look happy or sad, angry and excited, etc. My boys are learning to read people's expressions. I ask them if the person made the others mad, sad, happy, upset, angry, etc. I ask them if the person made a good decision or a bad decision. My boys are learning appropriate responses. I ask them what the person should do in that situation. My boys know to speak up, voice that the "pushing, name calling, not sharing" made them feel "sad, upset, angry". They learn to cope with situations through communication and finding help from another adult. They tell the person they need to be nice; they are mean; they are happy; they are excited; they like playing with the person; they like sharing toys with them, etc. They are learning to say please, thank you and show real compassion for others.
I took my boys to the park and a little boy lost where his mother was at. My four year old, then only 3 years, ran up to him, gave him a hug, told him it would be okay, and pointed to where his mommy was. He stayed with him and hugged him until his mommy was there. The little boy looked comforted and not scared anymore. It was the sweetest thing I have ever seen. It made me realize that we can teach children compassion. My son was so excited and happy that he made the other child feel good. He was full up spirit. He knew he made a difference. I think we can teach every child ways to make better decisions and how to help give comfort and compassion to others... even if we can't fix the situation.