Parts for Genestealer hybrids:
Gor heads (with their goatees trimmed off and the space for the horns filled in with green stuff) make for a good basis for hybrid heads, due to their elongated snouts. Similarly, the legs have similar jointing patterns as genestealers; though you'll probably need to green stuff some exoskeletal segments at the joints.
Lieutenant Varras, from BfM, makes for a good, classic 4th-gen hybrid, with his raised collar and unusual weaponry.
The two unarmoured cadian bodies from the tank sprue make great bases for generic imperial-style clothing.
You need to make the models looks sneaky and deformed. Bizarrely, necrons' hunched bodies and crooked legs should make for a decent skeleton to sculpt cloaks over for earlier generation hybrids.
Work on your green stuff skills to constuct hoods, ribbing and raised collars on current models.
House Cawdor and House Delaque gangers from Necromunda are perfect fourth generation hybrids, with just a little adjustment.