When I was referred to IAPT it began with a one-on-one assessment with a psychiatric nurse. We spoke for about an hour and then she sent a detailed statement of her assessment to my GP. This outlined my problems (e.g. low mood, suicidal thoughts, etc.), areas for concern (e.g. self harming, excessive alcohol consumption, etc.), and her recommendation for what treatment I should receive.
After the assessment she spoke with a doctor who recommended an anti-depressant that might be appropriate for me and I was put on a waiting list for therapy. A few weeks later I had a telephone assessment to see what kind of therapy would be appropriate for me and I was put on a waiting list for cognitive behavioural therapy. I was told the wait would be about three to six months. It took five months to be seen.
Whilst I was waiting for CBT to begin I started monthly visits with a psychiatric nurse to make sure that my condition was monitored. These visits lasted for about fifteen minutes and consisted of me telling the nurse my thoughts and my progress with the medication. I was asked if I had self harmed at all, if I felt suicidal and if I had any plans to act on those thoughts. I was also asked if I ever heard voices, if I thought I could read people's minds, if I thought I had special powers, and if I knew what day it was, what the date was and where I was at that particular moment (i.e. which hospital).
It's great that you've started to seek help for your problems but you will have to be patient. Receiving help for mental health problems can be a slow process but you're doing the right thing.
In the meantime you might want to have a look at this site -
http://www.getselfhelp.co.uk/. It contains a lot of helpful information that your therapist will probably cover later on.
4 week wait for mental health seems crazy to me.