What effect will a divorce have on a 4 year old ...

If you explain to the child that you and his daddy are no longer together in a way that the child understands there can be little or no effect on the child but dont blame it on the outher person or speak ill of him/her at least where the child can hear it as he/she is soon to be an ex. To stop a divorce dont sign any papers prior to or during the proceddings but if i may say contact a attorney in this matter to keep it legal. But if i may so say why stay in that type of relationship as he/she wants a divorce as this is no way to live just move on.

That depends on what kind of environment the child has been in until now. If there has been a lot of fighting or violence, a divorce is likely to improve the child's security. In any case, it might be a good idea to find a child therapist who is accustomed to helping a child of that age to express him or herself to work with the child for a few sessions.

You can't "stop" a divorce if one spouse is truly determined - it's stupid to try. All you can really do is try to work things out for the best.

I came from a divorced family and my parents got divorced when I was 8. The way to help a child any age child through a divorce is to explain it in an easy way for them to understand. Make sure they understand that under no circumstances was it their fault and make sure to remind them that you love them no matter what happens. A young child is kind of hard to explain whats happening so they might get confused or misunderstand. You just have to be careful what you say. Divorce can be hard on a young child like that but living in a bad situation where the marriage uisn't working can also be disasturous. If you are getting a divorce try to make the process as easy and pain free for your child as possible. And the only real way to stop a divorce is to try and work it out. If thats not possible and you know trying to get through it won't do any good then there's no reason to stay in the relationship.

You have been given some good advice already. It sounds like a divorce is coming if you want it or not. You may ask your partner if they will reconsider and if they will go together for counseling to make the marriage work for the sake of your child. It is worth a try. If they already have their mind set for divorce you can't make them stay married. You can make a divorce dirty or clean. The least you can do is to stand up for yourself and not just throw in the towel and give the partner everything including full custidy of the child which is as much yours as your partners. Do be kind as possible, but ask for the child or at least joint custody. Do get your own lawyer. One lawyer cannot represent your interests and those of your partner; some want to save money, but one lawyer isn't not a good idea. I have seen it tried many times and I can't recomment it. Don't badmount your paratner in front of the child; request your partner to do the same. Discuss with your partner and lawyers things such as that which would hurt the child. Children almost always blame themselves for causing a divorce. Don't assume your child hasn't figured out that he/she is the problem. Talk about it and assure the child that it is mommy and daddy who have a problem, not them. They are a little hard to convince. With much love and understanding children are able to handle most problems. If they show more rebellion than normal, or more anger,  or become withdrawn, you can seek a good child counselor to help them deal with their feelings. They then work through these feelings and come out OK. Each parent should not compete for the childs love with presents and anything to appeal to the child over the other parent. If you divorce please work together-cooperate-for your childs sake-even if you do not like each other. Your child is what is important. Best to you and your child. Hope for no divorce but deal with reality and a child's needs come before either parent.