I think I've seen them on Amazon but they're like 200 bucks (some dumb rare book algorithm). And if the amount added from the 3rd to the 4th is the same as the amount added from the 2nd to the 3rd I would say yes. But alternatively why not just download the RRG climbing app?
Also you don't really have to have 2 guidebooks. I only have the South Edition since that's where I usually climb. On the occasions I go the North I either borrow the guidebook from a friend or use the MP/RRGC app.
The most recent one is the 5th edition, so you are quite a few behind. And for full value you need a Miller fork guidebook in addition to North/South, so that's 3, not 2.
Would you be lost with your 2nd edition? No, the crags don't move around much. Would you missing out a lot of new routes? yes, tons, especially in the South region!
If you are strapped for cash, you can use the online info available on redriverclimbing.com to supplement your your old guidebook, beg to borrow books from other people at Miguel's, and so on. Or you can get The Best of the Red guidebook to get some info on better new routes and still keep it to one book.
I personally love guidebooks, and want to buy them, especially to support the excellent guidebooks that Ray Ellington puts out for the Red. I have seen my share of crap guides for various places, and they make me really appreciate how good we have it with guidebooks that have lots of topo photos and detailed maps So I would always opt to buy a guidebook, and I think the RRG guides are worth it. But that's just me.
Very much agree about guidebooks in general, and the one for the Red as well. The route descriptions are nice, the book is well-designed and convenient.
Plus, when you've been back home for a while without having the chance to go back, you can feed your nostalgia with it and/or plan your next trip there, whenever that is.
That being said, if you don't have 30$ to spare, you'll be fine without. A guidebook is nice and helps you make the most out of your climbing trips, but it's by no mean necessary to be a happy climber, imo.