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.

Yes it is recommended to have a slight gap underneath for hygiene .
In the top arch this gap should be minimal so it won't  affect the phonetics ,

There are several ways to retain an All on 4/6 type of implant bridge restoration. These include both cement retained and screw retained. The advantage of a cement retained bridge is that there are no holes in the biting surface that require filling and the bridge may not be as bulky. However, these are slightly more complicated to retrieve. With a screw retained restoration, retrieval is more simplified if the bridge needs to be removed for some reason. Either way there should be a small space in both options to be able to use a floss threader or super floss. Otherwise, a waterpik and daily oral hygiene would assist in maintaining the area.

Hello. I recommended you use super floss or water pick that will help you to deep clean all the area the brush tooth couldn’t clean. 

Beneath All on four prosthesis. It is important to have a minor gap  which can be cleaned easily using water floss or Implant brush specially made for the prosthesis cleaning. But the gap should not be too big that food particle is getting lodged and sticking consistently around the abutments and thus causing peri - implantitis. 
Also screw retained prosthesis is preferred and recomemded by expert implantologists as it is easily removed on regular basis let's after an year for follow up and mechanical or ultrasonic cleaning by the Prosthodontist in the clinic. 
Cement / Glued ones are tricky to be removed specially all on four ones. 
Hope this information helps. Good luck !

There must be a space between the All On Four prosthesis to be able to sanitize this area with an orthodontic dental floss or a waterpiker, only that this space should not be very large since if it is very large then it favors the retention of food, but neither can completely without space because then it would not be possible to clean that part of the prosthesis.