This puzzle is part of the Monthly Topic Challenge #8: Cellular Automata.
Find four capital letters that can replace the X's (and explain why).
I think the answers are as follows
L. Two blocks leaning left
I. Four Blinkers
F. Honey Farm
E. Devil laughing on his side?
Notice how it spells
Each of the clues is describing a configuration in Conway's Game of Life. The clues represent the configuration that the game settles down to if we begin with a configuration described by the corresponding number or letter.
Try it for yourself
I've been using this site: https://playgameoflife.com/
Title Explanation (thanks to Amoz for drawing my attention)
"Two Blinkers, A Pair of Boats" correspond to the numbers 4 and 2 and 42 is The Answer to the Ultimate Question of Life, the Universe and Everything in The Hitchhiker's Guide to the Galaxy