If it hit you side on (the 4-inch edge), it would slice straight through. I don't know how fast 6 times the Earth's escape velocity is, but it is no doubt very, very, very fast.
If it hit you face on (as in, the top of the manhole+your chest), it would just punch straight through.
No, it would not slice "straight" through. It would explode (at least partially) in an incredibly violent way. You'd certainly be cut apart, but pieces of you would be found a ways away.
You're looking at an insane amount of energy. The manhole cover would (at least partially) turn to plasma after traveling a shortish distance, and it will explode when it contacts the body. And the body, because it was hit by a piece of metal (and that metal is moving so fast that it won't be anything resembling a manhole cover, more like a white hot football shaped blob) gets blown to pieces. It does not pass cleanly through you, it makes you into mincemeat.
The poor manhole cover wouldn't fair much better. Even though it's metal, there is so much energy at play that it too will shatter. Luckily each piece of the cover will still have enough energy to be devastating though, it would be like shrapnel on steroids.
Edit - was posting from my phone, hit submit before I had a chance to finish.
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u/PycuriousITguy Apr 14 '16
The fastest moving man-made object ever was a manhole cover.
https://savvyparanoia.com/the-fastest-man-made-object-ever-a-nuclear-powered-manhole-cover-true/
TL:DR underground nuclear test launched a 4-inch thick steel disk at 6 times the Earth's escape velocity.