This came up in a match. I fired with Bossk at an X-wing equiped with R4-D6. My final results were 2 crits and 2 hits and his was 3 blanks. We had two ways that each ability would resolve during the damage dealing.
1. Bossk's ability changes the results to 1 crit 4 hits and then R4-D6 can cancel up to two hits. Meaning he loses 2 shields and takes a critical.
2. R4-D6 triggers first and therefor cancels nothing, then Bossk changes his results to 1 crit 4 hits. Meaning the X-wing is destroyed.
R4-D6 When you are hit by an attack and there are at least 3 uncanceled [hit] results, you may choose and cancel those results until there are 2 remaining. For each result canceled in this way, receive 1 stress token.
Bossk When you perform an attack that hits, before dealing damage, you may cancel 1 [crit] of your results to add 2 [hit] results