So i have R4-D6 equipted, opponent rolls 4 dice. Goes hit, hit, hit, blank. He rerolls, hit.

Do i get to cancel initial 3 hits, so 1hit for 1 stress, or do i get to cancel 2hits, for 2 stress before rolling defensive dice?

You do not get to cancel anything before rolling defense dice. R4-D6 triggers if you get hit by an attack, so you have to roll defence and compare result first.

For R4-D6 we have the ability of:

When you are hit by an attack and there are at least 3 uncanceled <boom> results, you may choose to cancel those results until there are 2 remaining. For each result canceled this way, receive 1 stress token.

Look to the sequence of the Attack phase.

Declare Target
Roll Attack Dice
Modify Attack Dice
Roll Defense Dice
Modify Defense Dice
Compare Results
Deal Damage

You are not actually "hit" by an attack until the Compare Results step. At this point, if there are more <hit> or <crit> results on the attack dice than you have <evade> on the defense dice, you are hit by the attack. So if you entirely wiff your defense roll, you may still end up with <hit> results that you can cancel with the droid. However, <crit> results are not affected by the droid at all.

Ubul and Slugrage have the right answers.

So i have R4-D6 equipted, opponent rolls 4 dice. Goes hit, hit, hit, blank. He rerolls, hit.

Do i get to cancel initial 3 hits, so 1hit for 1 stress, or do i get to cancel 2hits, for 2 stress before rolling defensive dice?

Here's an example:

(1) Your opponent rolls 4 red dice and gets [boom] [boom] [boom] [blank].

(2) You have no way to modify your opponent's attack dice.

(3) Your opponent spends a target lock to reroll the blank result, and gets another [boom] for a final result of [boom] [boom] [boom] [boom].

(4) You roll 2 green dice and get [evade] [blank].

(5) Your opponent has no way to modify your defense dice.

(6) You have no way to modify your defense dice.

(7) You and your opponent compare results. Your [evade] cancels one of his [boom] results, leaving [boom] [boom] [boom] uncanceled. You are hit by the attack and there are 3 uncanceled [boom] results, which triggers R4-D6.

(8) You can choose to use R4-D6 to cancel [boom] results until there are 2 left. Here that means you can cancel 1 in exchange for taking 1 stress. You do it.

(9) There are 2 uncanceled [boom] results left, so your ship suffers 2 damage.

That's a good answer. I was eating my lunch and didn't want my fries to get any colder by typing more.

Oh i see. I though different, thank you very much

Also, to over clarify, be sure that there are three uncancelled {boom} results. If your opponent is has rolled {boom}, {boom}, {boom}, and {kaboom} and your Y-Wing has rolled {squigly} you may not trigger R4-D6 because there are only 2 uncancelled {boom} results and you need 3 in order to trigger R4-D6.

Edit: And here we have the reason why we very rarely see this astromech on the battlefield...extremely limited utility...

{boom} = {hit}

{kaboom} = {critical hit}

{squigly} = {evade}