how easily does glass with some girth, say 4 3/4", insert vaginally compared w/ other materials?
It depends on what you mean. Smooth glass toys have very little friction/drag associated with the surface, and even less with any lubricant. So in that sense, a glass toy would likely insert more easily than say a silicone toy.
If you are basing it on size, an equally sized glass dildo would potentially be harder to insert than a silicone of the same size because the hard material has no give whatsoever.
In general, glass is much easier and faster to insert than other materials because it is so slick. Now, if it's really huge it may be harder to insert but certainly smoother than silicone (like Kindred said
Yep, exactly like Kindred & ksparkles16 said - it's been true in my experience too. It slides with ease, often requiring no lube at all for me. However, inserting can be a bit trickier, especially if the toy is curved or has a larger head. In fact, I have 1 glass toy in particular that I love once it's inserted, but it's really hard for me to insert it comfortably - I rarely use it anymore because of that.