Is it vital to have a gap between the teeth & gums for cleaning? I have All-on-4 implants top & bottom fixed.

There is a specific shape that the all-on-four bridge should be designed so that it is easily cleansed (convex or flat) and is normally tight against the tissues. Normally the arch is screwed into place, but it "could" be cemented in place, but if cemented it should be a resilient cement (or temporary cement) so that it can be removed without destroying the restoration.

Hello, this type of All On Four prostheses are mostly made with convex surfaces that we call hybrid, imitating the lost soft tissues and facing the tissue. The screw holes of these retained prostheses are then closed with composite fillings and removed by the doctor if necessary.There may be a very slight gap so that the bottom of the prosthesis can be cleaned. Waterpik devices that apply water with pressureare mostly uses for this purpose.

After the placement of All on 4s, your bone needs to fuse with the implants (osseointegration). During this period a lot of bone remodelling occurs accompanied with some shrinkage of gum tissues. This may lead to the development of gaps in between the prosthesis and the gums. 
Screwable prosthesis are used worldwide along with All on4s as it can be unscrewed and cleaned once in few months and then screwed back again.