How do you mean flat grind? Some people mean a full flat, like from spine to edge is a constant taper while others mean partially down the blade, what many call a flat grind saber.
f you're ok with a saber grind and not a full, then the Fallkniven S1 might work for you.
Or a Becker Harpoon. The Nessmuk is a good knife too, just a tiny bit shorter than you're 4.5" requirement at 4.3". Also the Becker BK10 Crewman. The Harpoon is one of my favorite knives. I'm hoping they one day do a special edition in something like CruWear or even Magnacut.
The Spartan Harsey Nesmuk is a full flat grind made from 1095 CroVan at 5.125".
Their Damysus has more of a saber grind, and is a more of a tactical-utility knife.
They also have a couple others that are outside your price limit.
Toor Field 2.0. The Mutiny is a cool blade, it's said to have been designed for a specific fighting style, but it's still a good multipurpose stainless blade. Also the Raven, Crewman, or Valor.
Cold Steel makes some that might fit your needs. The Master Hunter is right at 4.5 inches and I think is a full flat grind. The AK47 is a bit more tactical/combat looking, but a tough blade at 5.5". The SRK compact has a 5 inch blade.