For those interested, there is a guide to refilling Canon iP4200 cartridges at WhatCartridge.
Covers refilling PGI-5BK, CLI-8BK, CLI-8C, CLI-8M and CLI-8Y cartridges for Canon Pixma iP4200, iP5200, iP5200r printers, MP500, MP800 and MP950 multifunctions.