Location: The reason time exists is so everything doesn't happen at once
Any steel part on a car can rust. The fact that rust isn't as prevelant on C4s as it was on C3s it probably due to the fact that most C4s were bought as toys and have been garaged since new, whereas C3s and earlier Vettes were bought to be driven and, before people realized the collectability of them, many sat outside in all kinds of weather.
Check the usual suspects: frame rails (especially where they kick up in front of the rear wheels), windshield frame, radiator support.
A world of difference between C3, and C4. I agree with Jeff I have heard of the windshield being a problem, and I would imagine like anything if it is parked outside, especially in the grass, it will rot out the bottom.
I hear more about the floors being busted out than rust as an issue. About the only thing that gets rusty on mine is the rotors.
There is a world of difference between a C3 and a C4. You really shouldn't find any major rust points on a C4 like the C3 has. I've never heard or seen any problems with the windsheild frame rusting on any C4's. It should be an easy car for you to inspect.
I looked at my 94 twice before I decided to buy it. It had spent most it's life in upstate new York, and the last 4 years by the Texas beach. It has seen the worst possible weather conditions and been neglected and abused its whole life.
The 1st time I looked at I said no way, And I moved on never plaining to look back. But a few months later I got an email from the seller saying he is willing to cut the price low enough where I was interested again. When I went back I asked if we could take it back to my shop to put it on the lift (too look for rust)
I spent a solid hour under the car looking of any signs of rust damage and could not find any. Even the exhaust pipe and under the spare where in good shape.
It was surprising because everything top side clearly had severe sun, salt & sand damage.
I believe it will be a long time before we start to address rust issues in the C4's.
Location: '94 Ragtop 6-speed & LT4 Coupe, Central Maryland
The first C4s are now, believe it or not, almost 30 years old, and I have NEVER seen, or heard of, any corrosion issues with a C4 chassis. This is no doubt due in large part to the fact that C4 steel components of the chassis and birdcage were galvanized inside and out at the factory.
When performing a pre-purchase inspection of any car, it's wise to look for structural corrosion. But I'd be quite surprised to hear that you found any on a C4 Corvette, unless its frame had sustained damage that had breached the galvanized coating and allowed corrosives to attack the base metal.
I love C1/2/3 Corvettes, but the absence of rust issues on the C4s makes them very attractive, by contrast, as used cars. Structural corrosion, especially in the birdcage, of C2/3 Corvettes can turn into a nightmare. With a C4, that worry is pretty much non-existent.
I have heard of C4 windshield frame corrosion issues in rare cases where a butcher replaced a windshield and scarred the frame with glass-removal tools, so this is worth looking for, to the extent that it might be visible.