What happens if I kill a creature enchanted with Murder Investigation by casting Flowstone Slide
You'll get 4 tokens, because that's the power of the enchanted creature as it last existed on the battlefield.

It doesn't die quite immediately upon having negative toughness; it exists on the battlefield for just a moment until state-based actions are checked. The overly detailed sequence of events is:


Flowstone Slide resolves, making it a 4/-2 creature, still on the battlefield.
As soon as it finishes resolving, state-based actions are checked, and since its toughness is 0 or less, it's put into its owner's graveyard.
The token has now died, so Murder Investigation's ability triggers.
State-based actions keep being checked; Murder Investigation is put into your graveyard and the token ceases to exist.
Murder Investigation's triggered ability resolves. Since the object it refers to ("enchanted creature") is no longer on the battlefield, it uses last known information to check its power - and the last known power of the creature was 4, so X is 4 and you get 4 tokens.