The maths behind 4 weeks
A 120-hour course over 4 weeks needs 30 hours a week, or about 25 hours a week if you started with some relevant background and move quickly through familiar sections. That's close to a part-time job's worth of hours, so it works best if you have annual leave, a gap between jobs, or otherwise cleared time.
People who try to do this around a full-time job usually slip to 6-8 weeks instead, which is completely fine — it's still faster than most people assume a TEFL course takes.
