Answer: Using P2[4] as AGND is not advisable as this does not provide a low impedance path to the internal AGND. There is a 16.2K resistance between P2[4] and the actual AGND inside the PSoC. Refer the diagram below that shows the PSoC1 Reference Generator.
There is an 8.1K resistor and a capacitor inside the PSoC that form an RC filter on the AGND. When the AGND Bypass option is enabled, P2[4] is connected to the AGND through another 8.1K resistor. Adding an external capacitor on P2[4] adds another RC filter on the AGND and improves the noise figures.
So, use P2[4] only to add an extra filter on the AGND. For a stable AGND to reference your analog input, use either the RefMux user module or the TestMux in a CT block and route the AGND to an external pin through an analog output buffer. Refer the below KB article for details of bringing out AGND to an external pin.
Bring out AGND in PSoC1