riddle - Puzzle House #1: Entrance Hall

So, after typing in the solution to the Calculator Bomb, I stepped into my house, and was immediately overwhelmed by the number of traps and puzzle-seeming things littered around the room. Whoever did this, they definitely pulled out all the stops. The door leading to the kitchen was locked, and all the other doors out of the entrance hall were so barred that I didn't even want to try to undo them.

I looked around for another note from the person who did this, and I found:

Hello again. Miss us? We thought the calculator wouldn't stop you. So, we've trapped your entire house with puzzles. If you make it all the way to the back of the closet in the bedroom on the top left, you'll find a button that will undo all of this.

Of course, the door closest to the stairs that led to the bedroom they were speaking of was boarded up. There was another staircase, in the back, but I'd have to get there. Which was much easier said than done, considering the circumstances.

Having done a few Escape the Room challenges, I solved a simple puzzle found underneath my carpet, which opened a box, giving me this paper. It read:

Some thought but one, though some had thought a few.

Even shark teeth break arks unless they're new.

Vexed arias play through Asia, the sweetest song.

Everlasting melody has gone moldy before too long.

Never seeing that the singe is already near,

Fourth, third, differences first, second, is the order here.

(end of paper)

The lock on the kitchen door had a keypad and spots to type in four digits from 0 to 9. There was also a warning on the lock.

WARNING: If the wrong combination is typed in, even once, this lock will explode, which will not be fun for you. The order you type the numbers in matters. Oh, and still don't try to call the police. We are watching...

I couldn't find anything else of use around the room.

Good luck. I hope you have it (again)

(This is my first word-based puzzle. I hope it's good!)

Clarification: The paper is the only thing required to solve the puzzle. The rest is flavor.

Hint #1:

In each line except the last, two words are the same, but not quite... and the last line says 'differences first'...

Hint #2:

Notice something similar about the similar words? Perhaps a location will give a clue...