That would be hemoglobin.
Here’s some info lol:
Hemoglobin is a globular protein made of 4 polypeptide chains(alpha1 and 2, beta1 and 2)(2 types of polypeptide chains) and heme groups. There’s a heme group(which has an iron atom) connected to each subunit so there’s 4. Oxygen attaches to the iron atom so a hemoglobin protein can carry max number of 4 O2.