My high shifter isn't shifting in gears 1-4 (5-8 are fine). I took it to a shop and they said I need a new shifter because it's in "really bad shape". No details or anything. Does it seem like they're pulling a fast one on me?
Normally, a shop will not lie on their customers, as they do want you to come back again. But if you don't know whether to trust them or not, you can try to show your bike to another shop.
The main reason most of the times in these cases is a sluggish and stuck lip or teeth in the shifter Check this video. It should help
What type of shifter is it? As there are several kinds (Friction, stem, twist, rapid fire...) and whether if a "Really bad shape" is the most likely cause will vary. With down tube frictions shifters for example, they need almost no maintenance (Besides tightening the screw now and then) but with twister shifters it is common for the internals to wear out and not being able to stay on place. As mentioned in another answer if you don't trust this bicycle store, take your bike to another one for a second opinion and get things clarified.