You're dungeon delving. Or traversing the wilderness. You are also playing a game with an assumption of high stakes and danger (also known as high lethality). You have four players at the table. How the hell do you expect to survive an encounter?
It's true, in standard B/X or Original D&D rules, a dungeon room has a 1-in-6 chance of a monster (though you could turn more things into enemies), and each encounter has a reaction roll, with even with a hardcore referee, will be mindlessly attacking about a 28% chance of the time (that's 2d6 <= 5, and I would not put a roll of 5 to be mindlessly attacking, though the most hardcore would). That gives a cumulative chance of 5% of a guaranteed fight per dungeon room. Okay, random encounters double that. You will encounter a fight on average 10% of the time, with the harshest of referees.
But if you have a thief, magic user, and two fighters, you have 13 HP total, on average (using variable hit dice, 14 if not). Many monsters can hit for 6 on a lucky damage roll. Sometimes more. How the hell do you expect to survive an encounter??
Kill them before they kill you. OSR combat is swingy, and that's the danger. Hits don't land often (<50% of the time), but when they do, they hit hard. You need to use that to your advantage. The more attacks you have on your side, the higher the probability of hitting first is. At low level (and even up to a few levels), first blood is often victory. You need safety in numbers. You need hirelings.
Hirelings are not there to absorb damage. If you obviously use your hirelings as meat shields, the survivors will catch on and simply abandon ship. Then you're back to square one. You want hirelings that can hit something; you're a party, after all.
The PC's are leaders, not the commando force. Their role is to direct, and maybe throw in a hit if you're strong and well-equipped. You don't let the commanders fall; the party falls apart.
You Need Hirelings. They should even be the bulk of your party. If you want to survive in danger, you need to strike fast, hard, and often.
Confer also against YE COMPLEAT MANUAL OF UNMANNERLY FIGHTING.