The short answer
All-on-4 and All-on-6 use the same basic idea — a full arch of fixed teeth supported by a small number of implants placed in the jaw. The number in the name is simply how many implants support that arch. Four is often enough; six is used when your case benefits from the extra support.
Why would you need six instead of four?
The decision usually comes down to bone. All-on-4 was designed specifically to work with the bone most patients already have, using two angled implants at the back of the jaw to avoid areas of bone loss. If a 3D scan shows there's still not quite enough bone density or volume even with that angled placement, or if your bite naturally puts more force through your jaw (for example if you grind your teeth), a dentist may recommend a fifth or sixth implant to spread that load more evenly.
It isn't a matter of one method being "better" in general — All-on-6 isn't an upgrade you should ask for by default. It's a response to a specific anatomical situation, confirmed by imaging, not a preference.
Does the extra implant change the result you feel day to day?
No. Whether your new arch sits on four or six implants, the finished bridge looks and feels the same in your mouth — you can't feel the difference through the gum. The only real difference is under the surface: how the biting force is distributed, and how many points are anchoring the arch to your jawbone.
How the decision actually gets made
This isn't something you should have to guess about yourself. A proper 3D scan (usually a CBCT) shows your dentist exactly how much bone is available in each area of your jaw, and from that they can tell you honestly whether four implants will hold long-term or whether a fifth or sixth would meaningfully improve the outcome.
If you're comparing quotes from different clinics and one insists on six implants without having reviewed your scan first, that's worth questioning. The number of implants should follow from your anatomy, not from a fixed package price.
In short
All-on-4 is the standard, well-proven starting point for most patients. All-on-6 exists for people whose bone density, bite force, or bone loss pattern genuinely calls for extra support. Neither is inherently superior — the right one is whichever your scan supports.