TEFL courses that last 4 weeks or longer in an intensive format aim to give you the best possible grounding in TEFL/TESOL methodology and teaching techniques, as well as some observed classroom practice, before you embark on a career as a TEFL/TESOL teacher, make a career change, or start on a long gap project. This is the kind of course you need if you are intending to teach long term, will settle for only the very best employment prospects possible, or are simply a stickler for the most intensive course you can get.
The course options listed below are all quite similar in terms of course content and what you will be prepared to do by the time you finish. Two of them - Cambridge CELTA & Trinity Certificate TESOL - are externally validated by UK-based Teaching Awards bodies, while SIT TESOL Certificate courses are similarly validated in the US. This external vaildation, it can be argued, gives them more integrity and could make them better received by schools looking to recruit new teachers.
The remaining 4-week courses are courses created by other bodies which have identified a need in the market. They are largely based on the original 4 week CELTA and CertTESOL model, with 2 to 3 hours of input sessions and 2 to 3 hours of teaching practice each day. They may well be taught by tutors with very similar qualifications and experience as CELTA and CertTESOL tutors. Some providers also have their own internal validation process. These courses are generally very well viewed in their local context, and are often an excellent alternative for would-be teachers who intend to get a few years' teaching experience in a particular location. It can be argued that after a few years' as a teacher, the actual TEFL course you followed at the beginning of your TEFL journey is much less important than the teaching experience itself.