The correct answer is option 'b' (For).

Explanation:
The preposition 'for' is used to indicate duration or length of time. In this sentence, the players have been playing for a specific amount of time. Let's break down the sentence to understand it better:

- The subject of the sentence is "The players".
- The verb is "have been playing", which indicates continuous action in the past.
- The phrase "for four hours" provides information about the duration or length of time.

Prepositions:
Prepositions are words that show the relationship between a noun or pronoun and another word in the sentence. They often indicate location, time, direction, or manner.

Usage of preposition 'for':
The preposition 'for' is commonly used to indicate the duration of an action or event. It is used to show how long something has been happening or how long it will continue.

Examples:
- She has been studying for three hours.
- They have been waiting for the bus since morning.
- We will be on vacation for two weeks.

In the given sentence, the preposition 'for' is used to indicate that the players have been playing continuously for a period of four hours. It shows the duration of their playing activity.

Other options:
- Option 'a' (Since) is incorrect because 'since' is used to indicate a starting point in time, not the duration of an action.
- Option 'c' (By) is incorrect because 'by' is used to indicate a deadline or a specific time at which something will happen.
- Option 'd' (With) is incorrect because 'with' is not used to indicate duration but rather to show association or accompanying something/someone.

Therefore, the correct preposition to use in this sentence is 'for', indicating the duration of four hours.