System always used DOT 3, new replacement Master cylinder uses DOT 4

Changing the master cylinder in my Van listed above (g20), the whole system always used dot 3. So I filled the new reservoir with dot 3 and when I was about to mount it on I went to put on the cap and it says use DOT 4. Why the change? The part is identical, and I am assuming because it is newly manufactured it uses dot 4, but will DOT 3 be okay in this case? For its entire life up until this point its always been dot 3.