For the 8255 all reads and writes are byte wide. A write to Port C will only affect those pins whose output drivers are enabled for output, in your case only the lower port will be affected. The read operation will still read all 8 input pins so your software needs to ignore the lower nybble since it will contain the last nybble output. There is no specific command to read or write Port C nybbles.
So, if you have some device outputting, say, 0110 hooked up to the upper Port C and then write FFh to Port C a read from Port C will get 6Fh if you have Port C configured properly (control word 4,6,12,or 14).

Hal