All of Fallout: New Vegas, its quests, dialogue and factions untouched, played the Fallout 4 way. The Fallout 4 parts come from your own copy of Fallout 4, read on your PC when you first press Play.
Fallout 4 gun handling. Hold Left Alt to sprint (it drains Action Points). Aiming down sights is tighter, hip fire wider, every gun kicks and settles back by type, and reloads are faster.
Fallout 4 VATS. Your VATS key slows time instead of stopping it. Pick a target (A/D) and a body part (W/S) with a live hit chance, click to queue shots against your AP and E to fire them in slow motion. Hits fill a critical meter; when it's full, Space spends a guaranteed critical.
Weapon mods the Fallout 4 way. Every workbench gets a Weapon Mods list: receivers, barrels, magazines, frames and capacitors for 34 base-game guns, plus New Vegas's own 50 mods, all built from components and gated by Repair.
Scrap. Workbench > Scrap breaks junk into Fallout 4's 31 components (Steel, Screw, Adhesive, Spring...). Junk that Fallout 4 also has scraps exactly as it does there.
Fallout 4's interface and sounds. New Vegas's HUD and Pip-Boy fonts are rebuilt from Fallout 4's, with a VATS readout and critical meter on the HUD. Gunfire, reloads, VATS, Pip-Boy, quest and footstep sounds come from Fallout 4.
Sharper textures. The Mojave's ground and buildings are upscaled on your GPU in the background and switch on from your next Play.
Single player. You need Fallout: New Vegas (base game is enough) and Fallout 4. Melty installs xNVSE; the first Play takes 10-30 seconds to build the mashup from your two games.
