If it is a parallel output ADC, you can just wire up the top 4 bits. For instance, only connect your micro to data bits 9:6. Leave the others floating.
If you read the data into a variable (from an integrated part, etc), just bit shift the data by 6 bits.
It's even easier than that, you can set the 10 bit A2D so that the highest 8 bits are in ADRESH and the lowest 2 are in ADRESL. This allows you to only use ADRESH as an 8 bit A2D.
Simply SWAPF the top and bottom nibbles over, and ANDLW it with 0x0F to mask off the upper 4 bits - result a 4 digit A2D result in the lower nibble of W.