The bike is probably mostly fine, and may only need a 30 second adjustment, or it may need a new chain and rear gear cluster. But even that is common and not a disaster.

Chain wear and chain rust are two different things Chains shouldn't go 4 years without being oiled. A light oiling will fix the rusty look. However if you have been riding without oiling the chain that may have caused it to wear out. You should take it to a bike store and have them look at it. Maybe they'll fix it on the spot. Then learn basic maintenance, or make a habit of getting a tuneup at a shop once or twice a year.