You are interviewing them as much as they were interviewing you. Your response doesn't have to be any more detailed than the one they would give you had you no

You are interviewing them as much as they were interviewing you. Your response doesn't have to be any more detailed than the one they would give you had you not got through the technical interview. Just a simple & straight forward couple of lines thanking them for their time and that it was good to get an insight into the role however it's not currently right for what you're looking for. At the end of the day they gave you a decent look at what working there would be like, be glad that their interview process revealed what you needed to know. It's MUCH more costly to hire you then have you leave after 4 weeks. They should be grateful you are upfront and not waste further time & money pursuing a role you are not interested in sticking with.