Is it possible to reassign the numbers for note values? I prefer to have 4=crotchet. Can’t get used to ‘6’ being the number.
Yes is possible. You can customize this under: Preferences/Key Commands and search for “set note duration”.
But I suggest you generally to try to get used to the Dorico default key commands, because they are very much thought out and also consistent throughout the program. For example: little number=shorter notes, big numbers=longer notes 
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The default Dorico key commands for note durations (6 = crotchet, 5 = quaver, 7 = minim etc) are also used elsewhere in places where you can’t change them. For example, in the tempo popover to specify metronome marks.
Do try giving the Dorico defaults a go for a little while, and see if you can acclimatise, as you’ll almost certainly find it easier in the long run to keep track of one set of numbers rather than 2!
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If you’re regularly using a variety of notation apps, then it might make sense to ‘standardize’ your numbers (amusingly/annoyingly, Sibelius, Dorico and Finale all use different values, so which of those apps you designate as the gold standard is up to you… )
It maybe takes about two weeks to break any ‘muscle memory’ and get accustomed to the new values.
Otherwise:
“Grandad, why do you use those key values?”
“Because 20 years ago, I used to use this other app…” 