There's a file called "Gamelog.txt" in your Dwarf Fortress folder.

The folder can be accessed by going to your Steam library, right clicking Dwarf Fortress, clicking "Properties", then clicking "Local Files", and finally clicking "Browse.

You still with me?

Either way: In the "Gamelog.txt" file, you should see one or more sections of text beginning with the sentence:

Generating world using parameter set (parameter set name/numbers go here)

Now, below this sentence, you will see each of the multiple seeds that compose your world.

There should be four in total.

Count them, I'll wait right here.
No, go ahead, take your time.

You done?
Great.

You need to use the same worldgen preset to get the same world to generate when re-applying seeds.

And as far as I can tell, you can only input seeds when using the "Advanced World Generation" option.

Seeds for worlds made with the "Basic World Generation" option the game defaults to will be very hard to properly re-use, unless I'm missing something important, which I probably am, but hey, at least I tried.