some of the instructions read pins and some read latch, 27238302.pdf on intel.com page 117. whenever u read the port pins are sampled and present logic levels appear on the read. if u write '1' to any bit of port 0 pin goes to high Z state. write 'one' to the four bits u want to use as i/p pins and ignore whatever comes on a read (like 'mov a,P0') on the bits u are using as o/p pins. further more always write 'ones' to the pins u are using as input pins and data to four o/p pins.

abhishek