Is $4000 a good salary in Bangkok?

Yes you can live here on $4,000 a month with no problems on that offer, others who tell you it will be hard don't really know about living here. But why move if you can make the same money in your country. Unless you have a deep love for Thailand and it's people, I would stay where you are. Also keep in mind that when your child get to school age that international schools will cut into that salary.