For PIC16 how do I switch back to GPIO pin usage from SPI?
It seems that what you want to achieve is to use the standard pins of the SPI1 peripheral, except the SDI1 pin (since you specified pin RC4, which corresponds to the SDI1 pin). Consider the following extract from the PIC16F8877 datasheet:



From this extract it seems that if you make SSPEN = 0, then it would disable all of the SPI peripheral's pins. Do you mean that you ONLY want to disable the SDI1 pin, and use the rest of the SPI pins (SCK, SDO and SS) as per normal through the SPI peripheral? If that is the case, I am not entirely sure if that is possible to achieve (I have never tried it myself).

However, you do not make any mention of the TRIS register settings. The datasheet specifically mentions that even though you enable the SPI peripheral, you still need to set the appropriate TRIS register settings (see section 31.2.2 in the datasheet). Did you configure the TRISC4 register bit (assuming pin RC4 is the pin in question) as an input?