A good start is the BEAMA guide, download from here:-

www.beama.org.uk/.../guide-to-forms-of-separation--2011.html

Thank you Roger,  I read that, but I cannot gey my mind around as in when would you use from 4 over form 3b. 

I think the main difference is that Form 3 all the terminals are in the same space, not really separated e.g. like an MCB/MCCB panel board. Form 4 generally has the terminals shrouded better or divided better (ignore F4T1)

But from a practical point of view how does it work? does form 4 has the circuits separated as well and can be isolated, working on individual circuits without interrupting the others, whereas 3b you need to shut all the circuits when you work on one?

But from a practical point of view how does it work? does form 4 has the circuits separated as well and can be isolated, working on individual circuits without interrupting the others, whereas 3b you need to shut all the circuits when you work on one?