Since my answer isn't getting any upvotes (perhaps to long), let me give it a little bit of an edit, cause I thought I made some good points.
And, I'm going to skip the very possible "you would most certainly die". If a 4th dimension opens up, our blood, which pumps under considerable pressure, about 1/10th of an ATM or more, which, if an open dimension just appears, our blood would fly out in all directions and we wouldn't live long. There's also the possible that the gravity or the atmospheric pressure of a 4D world would crush us as well, or the wind would tear us apart or stretch us painfully. New dimensions are like that. You never know how the laws of physics will feel to our somewhat delicate bodies, so yes - instant death is a very possible outcome, but lets pretend we have a inter-dimensional suit that protects us and we can go to the 4th dimension and take a look around.
Our brains are also well designed to orient what we seen. In my room there's a lamp a few feet ahead of me. If I turn my head, the lamp still appears upright, not sideways. Our brains in combination with our inner ear, tell us which was is up.
The "screens" in the back of our eyes get the images upside down but our brains re-orient them to right side up and give us a sense of depth. We see up-down and left-right but also, partially interpret them and we interpret depth.
There's no right way to do it, as there's no clear cut laws of physics for a 4D world, but if we imagine things behave similarly. Light is a wave that becomes a particle upon interaction. Gravity is a field, we can assume that in 4D, light emits from light sources, dropping off by the inverse cube (not inverse square) in brightness. It's possible that a 3D being would be effectively weightless in a 4D world and immune to gravity.
How the receptors in our eyes would respond to 4D photons is more open to interpretation. If the photons start out as waves, which is basically what quantum mechanics tells us, the waves through 4D space interact with charges, it's theoretically possible that we D light could interact with our receptors. It's also possible that 4D light would pass right through a 3D being and wouldn't interact at all, so we might not see a thing. That's a perfectly valid and perhaps even probable answer to this question. But it's more fun to imagine what it would be like if we could see, so lets go with that.
So, would would the 2D screens in our eyes see in a 4D world?
From the point of view of the viewer, there's always just one depth dimension, and polar coordinates work better to understand what one sees than Cartesian coordinates do.
So in the 4D world, there's a depth dimension (which we interpret more than we see), there's up-down (which would be thrown off if we don't see gravity), there's left-right and there's Other left and other right.
A 2D person wouldn't be able to turn other-left or other-right, but they could be turned by a 4D being, and the 2 D screens in their brain would only see 2 dimensions out of the 3. They'd never see more than a single slice, plus any depth they could interpret.
In the 3D world, solid objects have shapes. In the 4D world, objects would appear to change shape simply by the person being turned.
If there was a singular light source which cast shadows, the shadows cast would be 3 dimensional. You could walk inside of a shadow, or walk around it and observe it's volume.
If there was gravity, and the person had a sense of up and down, the 3D person could see objects hovering in the air, as if by magic, because they wouldn't see how they connected to the ground and were supported, because we only see a slice of all there is at any one time.
Not unlike a single, unmoving Borg Polaron Beam Imagine if the beam is all you could see, you could interpret distance and get one slice at a time.
If you were correctly oriented, you could turn your head and see things disapear, not just turn out of view, but objects would change size as you turn or move in the 4th dimension.
Objects would also be so complicated they'd be hard to interpret. A 4D Square is confusing enough. Imagine a 4D chair or a 4D desk or a 4 dimensional book with 3 dimensional pages. Just dwelling on a 4 dimensional book or 3 dimensional shadows should provide some idea of how ridiculously difficult to interpret the 4D world would be.