I've heard that Level 10 4* weapons are roughly the same strength as Level 7 5* weapons. Is it the same for armor defenses?
Difference between 4* and 5* armor?
I'm not sure what the answer to your question is, because armor defense is so small and the effect of heat on armor is so small.
You might as well upgrade to get the added features of the 5-star armor. You don't really need to heat that 5-star armor, because heat affects defense so little. Or heat it to level 5, just to get the extra 1 health.
(Yes, I'm corroborating what Neometal said.)
To give a more specific scenario, I'm wondering because I plan to use BKM instead of BKR because I lack Radiants to max out a 5*.
In this scenario, would you recommend Level10 BKM or getting a Level1-5 BKR?
Looking at the bars, it seems like 5* star armor take off where the 4* star armor cap out. This is easier to see when comparing bars side by side. Like, the Ash Tail coat's bars and the Skolver's bars. The Ash Tail's normal defense bar ends near the 'N' of 'normal' meanwhile Skolver's bar starts in the half of the 'N'. Eh, if we had numbers this would be a lot easier to see... Anyways, this means that unlike weapons, 5* armor at level 1 has a little better defense than a 4* armor at level 10.
So, yes. Black Kat Mask at level 10 is fine. The only difference between the 5* version and 4* version is 1 point of HP and a small amount of defense. People usually go for BK for the buffs not the defense, though.
ETA: Err, if you can get the 5* BK Raiment, then do so. I thought we were talking about the Mask and Cowl only. Derp.
Since we are talking about black kat gear you should totally make it 5,* since as said by Strayed-Faris it's about the buffs not the defense (that's why the 3* bk set is nearly as good as the 5*), but if you have enough whiskers to make the BKR go for it.
Btw, the 5* bk items have +4 freeze instead of the 4* ones' +3 (if that would ever matter to you).
You're wondering about Black Kat suits --- the most exotic and expensive armor in the game? At 400,000 crowns per Margrel run (a conservative estimate, in my experience), BKR will cost you at least 2,400,000 crowns. So I'm surprised that you're worried about the cost of Radiants.
I have the money, but I don't want to grind/buy for Radiants. T_T
Thanks for all the feedback, fam. I will take your advice!
TBH get the BKR, you get an extra health bonus (when heated to level 5) if nothing else. And just eyeballing it you can see that the stat bar on the BKR at level 1 is the same as the stat bar on a maxed out Mail (Plus extra freeze res).
I actually don't know, but I'd assume yes.
However, there is no reason to keep an armor/helmet at 4* if you can upgrade it. Several items get new stats at 5* (chaos, vog, the 36 hundred gunner sets, etc) and frankly, the defense won't matter most of the times if you can dodge/shield enemy attacks.
(Same thing goes for some weapons which have better charged attacks at 5* (bombs, DA, etc), it's better to upgrade them.
And the Acheron is totally a must upgrade, iirc you only need to make it lvl 5 to be stronger than a lvl 10 Silent Nightblade.)
Plus, at lvl 5, they'll give +5hp and getting there is only 72 rads with 100% chance forges.