It's not impossible but will take very long time

For us it will take a long... loooong time :(

If I rember some threads are in shop update

Or there should be a coin system linked to SA. Less items and coins to spend in shop for higher tier mats, and maybe have the adversary of the current SA as buyable for coins too so heavier cashers still get that bonus- Karin and Shizune for previous SA and Deidara and Hidan for this one.

I'd rather spend coins on threads stead of coupons as coupons are hard to get

It's already coming later, when SA rewards get updated in the future.
But even on Hard SA, it gonna take some months to reach tier 4 armor, from lvl 3.
Well it's free at least & not 10k ingots. :lol

Just have some patience. :P

Is even upgrading from rank armor 4 to 5 even worth it? Feels like the scale won't be that huge.

will be nice to get other threads from SA

considering how hard it is to get 5 bloody threads ( pretty much event only) and that it takes 5 for just 1 point it will probably take months and months before any f2p sees lv 4 battle armour.