I dont see why it would take you 4 terms to do it? The NA placement is only 2 weeks- if you worked full time on your modules then you could do it in 3- no problem. Ive found the modules tend to take me about half of the amount of time they say it takes (or maybe Im just a bit slapdash!).
The admin part of the ITT centre at the OU is CRAP - its taken about 2 months for me to get a letter to send to me LEA to get financial support- but once you are on the course you very rarely deal with them and just deal with your tutor and mentor at school. If the school are happy to have you then you can arrange your own timings for your placements.
The thing thats really peeing me off with the OU at the moment is that I wil finish my course in Nov and theoretically I will have completed my PGCE then. However, because the award board dosnt meet until Feb, I wont get QTS until March. Which means I am delayed in getting a job. PITA
Apart from the admin side of things, the course has been fantastic though, I couldnt recommend it enough.