Don't start from mammals or chordates, because that implies late panspermia.
Instead, build a world (this is Worldbuilding, after all!) Have all major animals on this world have four eyes.
Quadrilateral symmetry would make this more likely, but "quadrilateral symmetry" is a slippery fish. You could have the symmetry last only for a time as they develop in the womb, as ours does: even mammals are arguably, at some very low level, quadrilaterally symmetric - two limbs above, and two below.
So if their symmetry lasted just a little longer, such animals could evolve into bipeds with manipulative arms, or animals designed to move quickly in one direction (our quadrupeds), and so on, but that their design is more quadrilaterally symmetric could inform your design of them, beyond the fact that four eyes was the typical pattern.
Prey animals would naturally tend to have the four eyes at four corners around their head, rather than paired. This would grant them stereo vision for 360 degrees. They wouldn't have to make the tradeoff that earth prey make, trading binocular sight for angular resolution. The rear eyes might migrate out on stalks/horns, the better to see around the body as they eat.
Predator animals would tend to have the rear eyes migrate forwards, up over the head or around the sides, granting them better peripheral vision for hunting, and perhaps being co-opted for other uses that the main eyes did not fill. Extended out on stalks to help with cooling, the extra eyes could be sensitive into the infra-red range.
Scavengers, opportunist feeders, would tend to vary between the two, and a state where the eyes were arranged in two pairs, front and back, could make a lot of sense, since it would still give about 360 vision, but binocular only "ahead" and "behind", which requires a whole lot less processing, allowing other parts of the brain to develop more, instead.
For bipeds, the ahead/behind distinction might not make much sense: they might have one direction in which they can move more rapidly, but if they can look in both directions, it's likely that their arms would be arranged such that they could manipulate items on either face of their body with roughly equal facility.
Perhaps one side would be more capable at fine work, and one more capable of hard physical work and fast movement. Perhaps one set of eyes would naturally lean towards long-sightedness, and one to short-sightedness.
Perhaps, like left/right handedness, they could be dorsal/ventral-favored, favoring one or the face (which could lead to various social complexities, biases, genders, etc).
Perhaps they would have a subtly different concept of left/right than we do ("on the left as you walk down the road" is unambiguous; "your left arm" is ambiguous to something facing in both directions).