brake fluid DOT 3 / 4?

[KZ]

It's time to change the brake fluid, and it says to use only DOT 4 but I'm having a hard time to find it. Tried several shops, all they have is DOT 3. Some older bikes use 3 or 4, so can I simply take 3? What's up with these numbers anyway? Are these

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[deckx]

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[KZ]

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[deckx]

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[Marco]

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[KZ]

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