Well, I
was just going to write about a couple issues with the G4000. But I kept looking deeper into the G0768 as well, and ended up with a pretty long post. Hope it's of use to you!
I've happily owned two different G4000 lathes (two workshops in two states), for a total of 10 years now. Sold one when I got a larger (12x24) lathe. +1 on armytbone's comments about the 4-bolt compound and the tool post. The 0768 looks like it has a more substantial compound mount, though according to the manual, you have to back the compound slide almost all the way off to get to the angle lock screws - could end up being inconvenient. The tool post is the same POS on both lathes.
And yes, the 4000 does suffer a bit from a not having a "reverse tumbler" (which the 0768 has - "reverse feed" on the photo). But there are lots of articles on the interweb about how to add one. I'd added one to the lathe I later sold. It's a fun project. I suppose I'll be adding one to my current lathe "one of these days."
A few more minor irritations on the G4000 (and their solutions) -
• The lathe is a bit low to the chip pan for my taste. Placed 1 ½" hardwood blocks between the lathe and pan. Also extended the bottom of the back panel, to help control swarf. The QCGB gear selector is now easier to use and cleanup under the lathe is easier. The 0768 doesn't have a QCGB, so this probably won't be an issue.
• The back panel has a gap where it goes around the motor (near the chuck, where a good deal of swarf is generated). No big deal, but unless you raise the lathe it can be a chore to keep the motor clean. I was able to kludge a closure. No idea what the 0768 is like in this respect, but I'd guess it's abut the same.
• The tailstock lock is just a nut, which requires you to keep a wrench handy. There are several articles on the web about adding a lever lock. I found a simple way to retain the wrench. Check the thread,
http://www.hobby-machinist.com/threads/lathe-tail-stock-lock-mod.57492/ My post, with photos, is at the end. According to the manual, the 0768 has a locking lever on the back.
• The 4-jaw chuck that comes with the G4000 is kinda crummy. You can buy a good one, but that's an added expense. The 0768 has what looks like a nice 4-jaw
• The spindle of the 4000 has a somewhat uncommon thread - 39mm x 4mm pitch. LMS sells threaded backplates. I haven't found anybody else who sells backplates with this thread pattern. With a threaded spindle, you have to be careful when running in reverse. The 0768 holds chucks and faceplate to a fixed backplate with three fixed studs, nuts and washers. This is good when running in reverse. But finding aftermarket chucks might be quite a challenge!
Some possibly negative aspects of the G0768 -
• There's no QCGB on the G0768. So you have to keep changing gears in the headstock ("end gears" in the manual). Might not be an issue if you don't plan to do a lot of threading.
• There's no 127 tooth gear. So all of the metric thread pitches will be inexact. Might not be an issue for short threaded parts, and wouldn't be an issue at all if you don't plan to do any metric threading.
• There's no separate power feed, as there is on the G4000. You have to reset the "end gears." This would be an important issue for me. I don't do very much threading, but I use the power feed very frequently.
• The G0768 is smaller (8" swing, as opposed to 9" swing), so it won't have quite as much capability/capacity (or power, for that matter). You'll have to decide how important this factor is.
Finally, you might want to check this thread on the G0768 (if you haven't already!):
http://www.hobby-machinist.com/threads/grizzly-g0768-8-x-16-lathe.36042/
.. might even want to PM one or more owners for their opinions, now that they've owned and used their lathes for a while.