Sure someone needs a level of intelligence and understanding to be good a teacher. I doubt whether the 4 week courses you are talking about (I assume you mean the CELTA or TESOL) have ever created a complete teacher regardless of educational background but they are still useful qualifications to gain and are far more valuable in my opinion than the weekend seminars or wholly online courses that lead to certification. Many moons ago when I did my CELTA the 2 trainers who guided me had a combined total of nearly 50years teaching experience. I would say I have a reasonable educational background and had taught Geography at a secondary school before taking the CELTA but I found it really helped to improve my language awareness. If you ask an individual in the UK who has completed their full time education in the last 20 odd years about parts of speech, tenses, etc. they probably wouldn't know what you were talking about because it is just not taught in schools anymore. A good TEFL qualification taught by experienced assesors can help bridge the gap between implicit and explicit knowledge of the English language.