Beating the game in less than 4 hours is not really hard. You have more than enough time to do that. The easy difficulty is the best option for this achievement.
The key is to use the manual save as often as possible. In case something goes horrible wrong or you need too much time for a section you can reload your last save and try again. Otherwise with the same mechanic you can reduce your time extremly. Save your game, explore the next section, find your best way through, then reload your save and do it in less time. With this strategy the 4 hours shouldn't be a problem.
Pausing the game will also stop the time counter so pausing the game is not an issue. But in case you die or use retry the timer will not be reset so only use manual loading of saved games. The reward for completing a 4 hour speedrun is a Circular Saw.
00:08 - Prologue
15:35 - Mia Fight (Guest House)
20:10 - Main House start
24:45 - Jack Fight in Garage
31:35 - Jack Fight in Morgue
38:45 - Old House start
43:00 - Marguerite Boss Fight (Old House)
46:00 - Marguerite Boss Fight (Green House)
58:00 - Testing Area start
01:03:25 - Barn Fight
01:05:25 - Lucas Birthday Puzzle
01:11:25 - Jack Mutated Fight
01:19:20 - Old Ship start
01:24:05 - Old Ship Videotape
01:34:40 - Old Ship Elevator
01:38:35 - Old Ship last section
01:44:50 - Swamp / Mine start (inside Saferoom)
01:48:05 - Mine fight up to the house
01:51:35 - Final Boss Fight
I have also unlock with this video the heal and item box achievements.