"It says [...]"

What is "it"? The manual for the SDK? The documentation for a sample program? Don't use "it" instead of giving enough information that other people will also have a chance (even if it requires them to own a similar SDK) to find the same text. We are not you. We are not sitting beside you, seeing the same things you see, or hearing the same things you hear. We only have the information you are supplying. And that is very scarce information - we have to drag every single little fact out of you.

A processor that supports push-pull will only place the output in push-pull when the program has so instructed. Push-pull is good when the processor pin is used as an output, but obviously not good at all if the processor pin is intended as an input.

If you are testing a sample program that is taking P4.0 as input and outputting the inverse on P4.1, then that sample program is obviously not using P4.0 as output, so the pin is not configured as push-pull. That means that you can connect the signal to GND or VCC. If it has pull-up internally or externally, then leaving the pin open will give the same logical result as connecting it to VCC.

On the other hand: a processor input signal is quite high-impedive. It doesn't take much current (sink or source) to make it register a logic low or high. That means that when experimenting with a circuit you don't know everything about, you can always use a 10k resistor to "short" with. The resistor is small enough that it will allow an input signal to go low or high, but large enough that it will limit the current in case you connect the resistor to the wrong signal, for example accidentally trying to short VCC or a processor output to GND.