I agree - it means the "entire compartment" however if there are 4 beds, you would not necessarily pay 4 X the fee. Here is an excerpt that I found on a website:
Many people mistakenly believe that you will have to pay four times more for such comfort. This is not entirely true. Russian Railways provides a discount when purchasing all seats in the compartment. Its size may vary, but the logic of the carrier is clear: servicing one passenger (fresh linen, cleaning) requires fewer resources than four. As a result, the total amount will be less than just multiplied the cost of one ticket by four, but, naturally, significantly higher than the price for one seat. This is a price to pay for privacy and space.