I have to correct the OP about model flag values at the end of the line, since this is a pinned thread with info that is different from my tests (read - "wrong" :D ).
There are 8 numbers or less in both categories, read from right to left (1st digit is most right one. I know. Make sure). Most left ones are omitted if they are zeroes. Each number is a sum of flags you want to set, so if vehicle IS_VAN and IS_LOW at the same time, you add 1 and 4 together and write 5. You can find all flags in the "/common/data/handling.dat".
After monetary value, first one is model flags.
40440010 - 0 means no flag is set for the category; 1 means ABS_STD is active; next zeroes mean it has doors and a single exhaust; fifth 4 means it's front suspension is MacPherson strut (it changes graphics a little); sixth is the same for rear suspension; seventh 0 means it's a car; most left number contains flags not used in the game (really, all four of them are deprecated).
4088 - 8 means it's big; 8 means random cars of this type will have ABS_ALT (see below for comparison), while others will have no ABS; 0 means it has doors; 4 means you can't look back in 1st person view; four more values are zeroes. "00004088" will have the same effect.
224009 - 9 means it's van and it's big; 0 means it has no ABS; 0 means it has doors; 4 means no look back (since it's a Yankee); two 2's mean it's front and rear suspension is solid.
About ABS values:
ABS_STD is a good ABS, it will hold your wheels just below their sliding threshold. You can see slight stutter of the wheels while braking in bad conditions. ABS_OPTION will install this type of ABS on random cars, leaving the rest with no ABS. Cars used to have ABS as an expensive option back in the days.
ABS_ALT_STD is unstable ABS. This one has lower check frequency, so you will slide for half a second, then brake for half a second, then slide again, and so on. More than that, it checks all wheels individually, so you will move side to side under heavy braking. Nevertheless, it's the only type of ABS I've actually seen on the streets in real life. And a funnier one, definitely. :D ABS_ALT_OPTION will add this type of ABS to random cars, leaving the rest with no ABS.
I haven't actually checked if you can have ABS_ALT as default and a good one as a random option. May be a good compromise.
About 5th and 6th digits (suspension):
AXLE_X_NOTILT seem to affect if the wheels camber change depending on deflection (you may have noticed that flying cars' wheels point inwards); AXLE_X_SOLID seems to be a stiff basic leaf spring; AXLE_X_MCPHERSON is a MacPherson strut; I can't say for sure what AXLE_X_REVERSED does. This flag changes suspension renderer shown in game. In my tests Feroci showed some weird black rods above the hood, when used with wrong suspension type.
Note about AI drivers:
AI drivers seem to have total control over their cars, driving with no wheel spin, no under- or oversteer, and never locking up brakes even under harshest of conditions. If they don't have the brakes to slow down enough, they will crash into the object, but never lock their brakes. ABS settings will have effect only on your driving. AI braking is similar to ABS_STD setting.
That's all I have for you today. New patch meant I can return to modding GTA IV, because I don't really like V.