There's a lot of life left in that 2011 for you, I'm sure. I think the upgrade to (at least) 8GB would be a great and inexpensive move, updating to Yosemite will be good as well -- you have to remember that WHATEVER the latest OS version is, a vocal minority (and I'm talking a very tiny percentage, each and every time) complain long and loud ... mostly about the fact that something has been changed/improved, or is just different, disrupting their comfort zone.
I'm not saying there aren't legit complaints ever (Lion in particular made some changes that were unpopular or not explained well, and mostly got changed back later -- and there's a current problem with iTunes 12.2.x that is pretty annoying), but the millions upon millions (scores of millions) of users that don't have any serious issues ... don't write to complain! So reading the Internet for information about how well something new is working is ALWAYS going to get you a lopsided and mostly-negative consensus, when in fact given a little time most of the complaints either dry up or end up being user error or both.
If you're having someone do the upgrades for you rather than doing them yourself, and especially if you don't use a great deal of hard drive space, you MIGHT consider spending a bit more to upgrade the hard drive to an SSD ... it makes a *tremendous* difference in how speedy the iMac feels, and brings its performance much closer to a brand new machine for typical use cases. It's not cheap, but its cheaper than buying a new machine and makes a big difference.