A fully heated 4* weapon does more damage than a new 5* weapon. Let's use the Faust as an example. How much do you have to heat the Gran Faust before it does more damage than the Faust?
A fully heated 4* weapon does more damage than a new 5* weapon. Let's use the Faust as an example. How much do you have to heat the Gran Faust before it does more damage than the Faust?
It's not straightforward, and depends on depth. Which is better between a level 1 5* and a level 10 4* will differ on whether we're talking depth 19 or 28.
This was a big topic of discussion when the Radiant drought was at its worst. You can find a bunch of threads by searching Google for "star heat weapon damage site:forums.spiralknights.com" (without the quotation marks).
For most weapons, it seems to be heat level 7 or 8, that the 5-star version finally surpasses the 4-star version. For this reason, you might want to wait on your 5-star upgrade, until you have a few hundred Radiants stored up.
Of course, there are often other differences between the 4- and 5-star versions, that might make you upgrade early. For example, Iron Slug changes damage type and Divine Avenger gets more projectiles.