Impant supported over denture in the maxilla with reduced inter arch space: suggestions?

Unfortunately you have insufficient interarch space for a bar overdenture. I would get her into a trial denture and see if you can open the posterior VDO a few mm if you can get 10mm in posterior you could do a bar overdenture splinting the implants but a fixed approach with a screw retained hybrid would be the easiest approach. A picture of her smiling would help with further suggestions

The number one reason All on 4's prosthetically fail is under-reduction at the time of surgery. Without enough room, the size of the bar and the bulk of acrylic must be reduced leading to bar and prosthetic fractures. Under-reduction = doomed from the start and will always be a compromise.

If you have minimal interarch space then you didn't follow the "all on 4" protocol. Classic example of why these extremely difficult restorative cases should be in the hands of specialists or highly trained generalists. There, that's airing my frustrations because, as a prosthodontist, I have to bail out soo many well meaning GP's and surgeons because of lack of planning. We do learn a thing or two in our 3-4 years of advanced training! OK, I'll quit bitching. Now, how many implants are there? If you have only 4 then connect with a very low profile titanium bar and use low profile attachments. I generally use a hader clip anteriorly and use the larger bredent balls off the distal of the bar, not on top. Then metal reinforce the overdenture with mesh, like an RPD. The marius bridge requires a bigger bar and more complex locking mechanisms so is difficult in tight spaces. How much labial undercut do you have? This may dictate your path of insertion. Hopefully you removed any cuspid or facial undercuts. If not, take this into consideration as you plan your bredent attachments on the posterior aspect. Another option may simply be using custom abutments and a crown and bridge restoration. Butt it against the ridge, using pink procelain or composite to create correct anterior tooth proportions. If you only have 4 implants and you're worried about flexion, how about custom abutments, a titanium framework then composite bonded to the frame (both tooth colored and gingival colored). this will allow flexion of the substructure without the layering material coming off and, even if it does, it is easily repairable. Sorry for my rant, but I hope I have given you some options to restore your patient. The last thing I want to do is remove implants but sometimes that needs to be done, along with adequate bone reduction, so the long term plan can be fulfilled. As always, the goal is a happy, well restored patient with a good stable prosthesis and a good long term prognosis. All the best.

Too little space for a Marius or a 2-in-1 or other similar designs. If the VDO is already correct, you really can't increase it in edentulous arches, even if there are implants. Options include a Ti-Zir hybrid like the Cagenix 360; titanium undercarriage with zirconia overlay is strong and therefore vertical requirement is less. You may also consider four individual crowns with a precision partial; this would fit the space you have and the flange of the RPD will hide some of the uglies. With milled guide planes and rests, the attachments (like a VkS) are very effective.