I would recommend to upgrade sasuke to 5* and farm fragments to refine for iruka or 9tail chakra naruto, if you don't use him a lot. Otherwise, just keep him at 4* and do skill breakthrough.

I've fully skill broken with both kirin and his chase and I love him. I use him in Ranked and in Matsuri challenge and am close to getting him to 5 stars. Its definitely worth it to get at least Kirin +1 before going for 5 stars because you get the added paralysis effect (otherwise just use Playboy sasuke). He's probably the best and easiest out of the 6 so far to fully breakthrough as I almost always get at least 2 sometimes 3 frags per 3 sweeps of an Elite Instance. As for farming Ninja Frags, I got Kabuto, Kankuro, Hinata, Anko, and Suigestsu to 5 stars and just get fragments through the shops. I don't know how many fragment points Sasuke is worth but Hinata, Kankuro, and Kabuto are all worth 10.

We only have half of his skillbooks now (we are missing his other 2 chases and standard attack) as well as skill mutations later on. Later on we should be able to modify skills the same way purification works- we get a random skillset from all available ones. Current breakthrough will set minimum, so lets take an example of his mystery: 0, +1, +2, y, y+1, y+2- if you make kirin +2 now, later on u wont get lower than that during dice roll.

If you can utilize his mystery during ranked/gnw and actually get that paralyze off, its worth to fully break him. It also increases mystery dmg by small margin (tested kirin +1 and +2 on a dummy, had about 10k hit with first and 10.5-11k with +2). I have all his current upgrades, and from my experience, if you cant get mystery to hit, theres no difference since we dont have dmg skillbreaks yet. However later on, with good chases he really starts to hit hard and might be a strong ninja, so current farming will profit in a long run. As for fragments, there are other ninjas that you can buy in redemption market that will contribute towards fragment refining, so saving his frags wont hit your refining that much. It's up to you, you need to decide if you want immediate points and few more scrolls/irukas/9tail frags or small power boost and easier skill draft later on. That is, if you will use him, if not then go for refining with no hesitation.

I personally think just 5* him first so that you don't need to think about the same issue when the next round of breakthrough comes.
Frags will always keep coming.

sasukes 1 frag is worth 10 points.

just 5* him, germany is getting skillbooks for free from daily sign in, they already had kakashi and sasuke the past 2 months and gave away their 2 mystery skillbooks for a full months sign on. china also gave skillbooks for free so i would wait and get them free if you can, then you save those points for iruka/naruto frags and seal scrolls

A lot of good options here.

Get him to 5* later we will get a sort of treasure run by tsunade where we will flip for random skill books so we wont need the frags.