I have recently tried both the VMS-1100 DVR and the VMS-4100 DVR. In addition to feature and performance differences (e.g. the VMS-4100 has: 4K support; faster menus; bluetooth remote with voice search and commands, instead of conventional IR remote with many buttons and spelled search), I also encountered some problems that perhaps others can learn from, and add their experience to the discussion. The info below is pertinent for many who are considering upgrading (or downgrading) their DVR.
The VMS-4100 would not pass through / encode / decode DD 5.1 surround sound channels properly, which some techs confirmed -- it deletes (at least) the center channel, which makes it almost useless for surround sound. Needs to be fixed, and hopefully will be fixed soon. More information from others with the same problem at: https://forums.verizon.com/t5/Fios-TV-Technical-Assistance/FiOS-TV-One-VMS4100-Audio-Pass-Through-Only-Passing-5-1-Channels/td-p/879766
I myself went back to the VMS-1100 for now (the tech was lucky to still have one in the truck) just to be able to have surround sound work right. But later I noticed some HD picture quality issues with the VMS-1100, where dark areas in particular had a lot of cloudy artifacts and banding. This is apparently much improved with the newer VMS-4100, but I did not get a chance to test it myself. I found more information at the following thread (among others): https://forums.verizon.com/t5/Fios-TV-Technical-Assistance/Poor-Picture-Quality-on-Quantum-TV/td-p/852596
If you have information to add related to the links above, then I encourage you to post there on the appropriate thread. If you have other points of comparison to share, related to your experience with VMS-1100 vs. VMS-4100, then I hope you will add your posts here, to help inform the community. Both units provide a lot of capability. 🙂