when I am not able to train because I am going to a conference or on vacation, I know it always well in advance. So I prepare for that time of absence with workouts that my clients will do on their own.
I don’t even bother to try to find a ‘replacement’. It’s not that I consider myself irreplaceable but I know every quirk of my clients, their preferences and past problems, and it is hardly worth trying to teach another person that, even if it was for a period of 4 weeks.
This has worked well for me for all my 18 years of training.