it's been multiple eternities since I've played 3 or NV. 4 I've played more recently, but that was ... basically when the last dlc dropped? So... memory of those games not really the sharpest.

London is mechanically almost identical to 4 (duh? because mod, duh.). The parts of the main-story I've done has been great. I recall preferring side and dlc missions over main-plot in 4, but I was pretty into the main stuff (dlc or not) in NV.

some thoughts about it in no specific order:

• I'm around 20 hours in, and it feels like I've only started. Plenty of map markers opened, but VERY little of actual land area explored.
• there seems to be less emphasis on settlements, haven't had a permanent residence (storage, bed) so far, been storing my junk in some friendly town crafting stations which don't seem to de-spawn stored stuff (as tipped in the mod's discord).
• but settlements are apparently still a thing (not sure how I feel about it, they were the worst part in 4, imo) - haven't aquired a settlement yet.
• it's bit more difficult than 4, ammo is a bit scarce and some enemies are ridiculously tanky, even at normal difficulty.
• the early game can be a bit brutal, but eases off as you get levels (ref: at ~10-15h or so in, I was finally able to take enemies down with confidence)
• The city is HUGE and dense (requires some more grunt from the computer than base-game), there's things to do and see in every corner, a lot more buildings to enter.
• "level design" is mostly great, but there have been places where the route forward/mission critical progression item was hidden by all the clutter and generally darker lighting.
• the level design also contains surprisingly deep puddles here and there on the streets, and those build up radiation QUICKLY. Bodies of water in this game are dangerous.
• Food and other non-stimpacks seem to be in quite a big role, at least in early game. IIRC fallout 3/nv/4 basically showered the player with stimpacks.
• Voice acting is pretty good, if a bit hammy at times.

I recall enjoying all fallout games, some more than others. I mostly mod for convinience (eg. busywork, carryweight) so I'm not really playing the game(s) as intended. And this one is great... if a bit buggy here and there - but not any more so than actual Fallout games. :D

I totally recommend London, but it comes with an asterisk: the mod requires tweaking, and depending on your computer-skills it can be either "man, this is obvious" to "wtf am I doing". Config-file editing and extra mods are required to get it actually working. Def recommending Mod Organizer 2 as well. Also, if your base fallout 4 (and dlc) are from steam, you need to downgrade from the "Next gen update". I'm fairly sure there's good step-by-step how-to's to get it set up at this point.