No, it does not work that. If you read the Glossary Text of Mettle you'll notice the +4 is only on a reroll of an attack or save:
"If this character spends 1 Force point to reroll its attack or save, add a +4 bonus to the result. If the character can spend Force points more than once per turn, bonuses from additional uses on the same turn are cumulative. "

That is incorrect Kezz. You only get the +4 when you use a force point to reroll something. Making a second evade save on the same turn does not qualify as using a FP to reroll something.

Now, GOWK on the other hand (or an evading jedi with MotF and mettle) can access mettle sort of like that.

Let's say Cad Bane is attacking GOWK. He fails the first SSM save, so rerolls with a FP at a +4 to the roll. 2nd shot, he rolls SSM at the normal 11, fails again, and uses a fp to reroll. This time the mettle stacks with the earlier instance of it, for a +8 because it's the same turn.

Another example would be in the case of an evading figure with mettle and a CE that allows rerolls. If you reroll the first failed save, with a FP (holding the CE reroll until later), and you fail the second one, you can reroll the third time with the CE, and still have mettle (since you already met the qualifications for that particular attack). This still won't effect later attacks, but it is a way to get the benefits of mettle more than once. Most people would do the free reroll first, and miss this opportunity, but there are certainly situations where it's the better way to go.

Something Mettle does work with though im pretty sure i remember seeing this, but you would need MOTF 2 or higher atleast, that with Cortosis Gauntlet, when you make the LS block save if you fail it & have mettle & MOTF that when you go to reroll you get the +4 added on so if you were to need lets say an 18 for Gauntlet, you would only need to get a 14 for it to trigger. This is mostly seen in Zayne Carrick which his could work either way with Karmic Mettle, so depending on if he rolls odds or evens for initiative you would determain the add-on or take away if you gave him MOTF 2 & were going to use k-mettle & C guantlet..

Rule.