The Mongoose Dolomite ALX is probably the only fat bike that fits your criteria, priced right at $500. It has 4" wide 26" tires. Mine, which is the 17" Small frame, weighed 36.4 pounds stock. The 19" Large frame is recommended for riders up to 6' 1" tall. The frame, fork, stem, handlebar, crank, and rims are all aluminum. I hesitate to call it "good", since it uses no-name components (drivetrain, shifters, brakes) which are far from premium, but it is certainly entry-level, and it's a decent enough platform if you want to upgrade things. So far, I have been unable to dial in my shifters/derailleurs so that they shift flawlessly through their whole range. Some have complained of "wobble" in the bottom bracket, but I haven't noticed a problem with mine.
As far as I can tell, the Dolomite brand is a Walmart exclusive, and you can find the ALX on Walmart's web site. If it's out of stock, just keep checking back, as they are actively restocking right now. Note that the ALX is the only one of the Dolomite bikes with an aluminum frame.
As for gearing, it really depends on how you intend to use it. Fat bikes aren't really limited to or targeted for specific uses. The ALX is a 16-speed, with a 11-34t rear cassette and 36/22t chainring, and so far I have found that is plenty low enough for muddy off-road use, but is still reasonable for getting around on roads. A common user modification is to change out the chainring to something they like better.