DOT 4 fluid for clutch- why?

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My avatar truck is an '02 K2500 with the 8.1/ 6spd manual. Noticed the other day the clutch reservoir was on the low side and *then* I noticed the cap said "DOT 4 only". I just had DOT 3 brake fluid on hand so I did a little research and made a phone call to a trusted mechanic- long story short I ended up going to town for a bottle of DOT 4. The brake system in this truck takes DOT 3 and I end up wondering why the clutch wouldn't use the same fluid?

(if this has been covered somewhere, apologies, the search results were inconclusive)