The difference is a factor of two.

I regret buying a 2.8. it is much more expensive and the new cameras are so good with high iso that you cant notice the noise anyway.

Two.

If you shoot indoors, you’ll likely want an f/2.8. It’s the difference between 1600 and 3200 iso or 1/250 and 1/500 shutter speed. I’d never shoot anything indoors (unless it was professionally lit) without f/2.8 lenses.

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If you’re inexperienced with shooting sports, I’d suggest renting a lens and trying it out. Some people who try to shoot sports/concert photography for the first time get very bad results because their settings aren’t right. Use the rental experience as a learning process on what equipment you should buy.

All depends on the available light where you are shooting.

F2.8 lenses are bigger, weigh more and cost a good bit more. Can’t tell you if it is worth it for a stop of light. Not enough information was provided.

One stop. The difference between 1/500 and 1/1000 second, or 6400 ISO and 12800 ISO. Plus double the background separation.

It’s noticeable, but the advantage you’ll get shooting sports is the faster shutter speeds you can get. If you’re serious about what you want to photograph it’ll be worth the extra money. Just make sure you’re going to use it enough to make it worthwhile. The last long zoom I bought I ended up selling because I rarely actually needed it.

if you dont know what is the diference, you should buy nothing…

The difference between being able to shoot at ISO 6400 and ISO 3200. Or ISO 3200 and ISO 1600. Or 12800 and 6400.

Depending on how your camera and sensor handles noise, this may be more or less meaningful to you.

I am no expert but I would say f/1.2 difference.

It’s twice the light. Useful for shutter speed and focus sensitivity. Focus speed and accuracy depend based on lenses in question. If you are shooting in daylight an f/4 will be plenty. At a well lit big event f/4 is plenty. As things get dark, fast action, and you are closer in(requiring higher shutter speeds to stop action of players) that’s where the f/2.8 may be needed. With digital, high speed ISOs are so good, that it often makes sense for the enthusiast to jump from 3200 to 6400 rather than spend another $1000 on a lens. If you are making a living with it as a primary lens, just buy the fast glass.

The longer your lens, the more shutter speed you want.

The more active the sport (say basketball vs golf), the more shutter speed you want.

The less light in the venue, the bigger f/stop you want.

All of those say F/2.8 over F/4. If you want to save money, give up a few millimeters on the long end. I bought a 70-180/2.8 Tamron, and I don’t miss that 20mm on the long end. The lens is a lot more compact as well. Also shoot raw, high ISO, and use really good denoising software. But it’s always better to have sharp, noisy photos than fuzzy, smooth photos.

What body, DSLR or Mirrorless? If DSLR, I would buy a clean used F2.8 with image stabilization. If Mirrorless, an F4 with IS will be fine.

My Minolta Beercan has a maximum aperture of f4 at which it is as sharp as a very sharp thing in a razor blade factory. But it weighs enough to give me the trembles.

the 2.8 means double the light from f4 but also double the price lol

My 70-200 GM II is still burning my wallet.