First, consider the domain structure, is it a 4 character domain or a pronounceable 4 letter English word. If it is an unpronounceable word or possible abbreviation, (LLLL.com & LLNN.com) the price can be easily around 4k+ to anywhere on top premium price brackets.
If it's just 4 character combination with brandable possibilities, (most of LNLL.com) I have seen them selling between 150USD to 2000USD.
I think the price of the 4 character domain depends upon -
1. The brandable proposition of those characters in the current market
2. Premium letter combinations
3. Previous purchase history
4. Domain Age (in some cases)
etc.
Please feel free to ask questions on 4 letter domain names. I love 4 letter domains and have traded LLLL.com & LNLL.com in the past.