Which Euybike models does this tire fit?

The 20 x 4.0 size fits the S4 and K6 Pro. The 20 x 3.0 size fits the F6, NXB and F7. Match the size marking on your current tire before ordering.

Does it come with an inner tube or wheel?

No. The package is one tire only. You reuse your existing wheel and inner tube, so replace the tube separately if it is damaged.

How many tires do I need for a full set?

One pack is a single tire. For both wheels you need two tires, and two inner tubes if you are replacing those as well.

Can I use it on another brand of fat-tire e-bike?

Only if that bike takes the same 20 x 4.0 or 20 x 3.0 size and the rim width suits it. It is designed for Euybike models, so check your sidewall marking first.

Is this an upgrade over the stock tire?

It is a stock replacement, not a performance upgrade. There are no tread or compound specs to suggest better grip or puncture resistance than the original.