I am four, but my second can be any of the five, though not sometimes.
- My first is final.
- My second often warns of danger.
- My third is what you are in the middle of right now.
- My fourth cannot be found.
- My fifth is what you desire.
What am I?
Bonus: Can you think of any others like me?
Since this has been answered already, I'll include some other examples below. Don't roll over the spoiler box below if you still want to solve the original puzzle on your own.
- BAD BED BID BOD BUD
- BAG BEG BIG BOG BUG (Thanks @Deusovi)
- BALL BELL BILL BOLL BULL
- BAT BET BIT BOT BUT
- FALL FELL FILL FOLL FULL
- FAR FER FIR FOR FUR (Also FYR, but that's a proper noun from the Elder Scrolls games, so maybe shouldn't count.)
- MASS MESS MISS MOSS MUSS (MUSS being an informal spelling of MESS, generally done to hair.)
- MAST MEST MIST MOST MUST MYST (Thanks @APrough, though as noted, MEST and MYST are both proper nouns.)
- PACK PECK PICK POCK PUCK
- PALL PELL PILL POLL PULL
- PAP PEP PIP POP PUP
- PAT PET PIT POT PUT
- TAN TEN TIN TON TUN