Mr. F may have an AP/Dura advantage here (around 2.7/8 Gigatons), but Luffy has a massive speed advantage, and his elasticity is simply FAR better. He'll be able to take hits even in base (considering WCI Luffy scales from taking countless blows from Katakuri) and unlike the cases where he has fought Haki users, Mr. F isn't going to reliably harm Luffy as his attacks are almost entirely blunt.
Luffy may take SOME damage from hits, but he's got massive advantages in every category: Speed (>>>>>Mach 909 in base, higher w/ Gear 2nd, higher with Soru, even higher with Gear 4th), Stamina by a ridiculous margin, Range by a large margin (25+km vs 4.8km), High resistance to Blunt Force Trauma (meaning Mr. F will deal almost no damage with each strike, considering the small gap between Luffy's dura and Mr. F's AP), Attack deflection (Gomu Gomu), etc with his DF powers.
He also has constant Future Sight like Katakuri has displayed, and his Haki increases his Base AP by x amount, so even if there's a large gap between Base/3rd Gear Luffy's AP and Mr. F's Durability, he can still hurt him at least by some degree.
Luffy takes this Low-Diff. He might not even resort to Gear 4th, as there's quite frankly no need here.