Not counting the new units and cardsystem, Space Hulk: Tactics faithfully follow the 4th edition rulebook. You can find it here:
The rules are mostly faithful. Some differences:
- the greatest difference are obviously new units (Apothecary and Biomorphs) and the cards: CPs and Blips are gained through them. Blood Angels ramp up close assault bonuses, ultramarines get much more APs and Deathwing Marines are especially able to increase their Librarian's psi points. Genestealer can break through with Bulwark biomorph card which forces a Marine to use 1 dice when fighting (which leads to space hulk 1e probabilities when applied to a storm bolter in overwatch). 0-Blips exist, remain only 3 turns and cannot enter a Marine's Line Of Sight. With 2 or 3 miasmic decks the Genestealer player can get 4 Stealers per turn or even more in average, which would amount to two or more of the original board game's blips.
- time is limited for both players, they can add remaining time in the next turn. Doesn't reduce if Marines lose a Sergeant.
- Genestealers don't have to pay APs to turn anymore as their facing doesn't matter.
- Genestealer can convert from Blips in Line Of Sight. A Blip can convert after spending APs. Stealer can be activated anytime after conversion with remaining APs.
- Squad APs instead of boardgame Command Points. Marines may activate as often they like and retain their APs.
- Psi Points cannot be used in close assault, instead the Librarian can channel his force axe for 1 PP +5 in close assault.
- Heavy flamer has 3x3 area of effect and decreased range.
- Storm bolter range is 12 squares in Overwatch and normal fire mode. Assault cannon range is unlimited in Overwatch and normal fire mode.
- Librarians psychic storm is used in 3x3 area of effect mode, no single target attack. No psychic power to gain a command point.
- Overwatch and Guard can both be active at the same time. Other actions can be performed while in any of the modes.
- LOS is based on the squares you actually see in first person mode. You can see slightly more squares than in the board game.
By the way, I uploaded a ruleset of "Space Hulk: Tactics" for the board game on boardgamegeek.com: https://boardgamegeek.com/filepage/171649/rules-and-cards-space-hulk-tactics