I mean sure. It's probably done in some extend. But on most posts it's just not worth the trouble setting up a whole bot network, just because you don't like to post of 3volver.
And you could build a similar bot network for pretty much every other social media platform, so it's not realy worth to invest that much time in altering the upvotes of a lemmy post. Lemmy beeing a network most people never even heard about.
The average person has to manually make all the accounts, switch between them, find the post and vote on it. Thats a lot of effort for one additional vote. If you realy want to have an effect you have to build a botnet and thats a lot of work to annoy some people.
My guess is probably what's happening with these 4-account people is they have them all open at the same time in different browsers, and since they only use them to upvote one account's post submissions they just have to follow that account.