It's still true that in Thailand if you pay peanuts you will get monkeys.
4000B is on par with the lowest legally paid jobs available, not pay for a job that requires responsibility, loyalty, experience, AND English skills.
4000B is what you pay someone who is disposable.
I'd say crank that up to 6000B for the first month, 8000B thereafter, with a 6-month review leading to incremental raises. That's what the Thais I know who have (and stay in) full-time jobs with good English skills are making to start in Bangkok.
Plus tips, freelance, whatever.