How long have you had it and how has it held up?

You'd really need the schematics to do the kind of customization you're describing. I think you can change out the crossfader for a better one on that mixer, but you'd be making circuitry changes to get more settings than you have on the 2-position switch.

you can mod the 05 with a stanton focus fader. If it has the same plug as the 05 cross fader, then in theory, you should be able to mod it with a Focus Fader. The focus fader is good for scratching. modding an 05 with the focus fader is very easy to do.

4. CAN I REPLACE THE EXISTING CROSSFADER WITH A ”PRO” FADER WITH ADJUSTABLE CURVE CONTROL?
The crossfader for the VMC-004 has exceptional smoothnessfor a mixer in it’s price range and you are able to replace it by either a standard Vestax crossfader CF-R; CF-RUS or the short way crossfader CF-S (20mm) But most important is that you can replace by the top level PCV-crossfader CF-PCV. Crossfaders with curve control are available on series 05; 06 and 07.

Yes, you can mod it with the focus fader, or other cross faders designed for scratching.

if you replace it with a pcv fader, it won't make a difference. The curve adjustment is on the mixer, not on the crossfader itself. And since the vmc-004 doesn't have curve control (if I'm not mistaken that is), you won't get a sharp curve on the pcv. You could a-mod or scrape mod the pcv, that will work. Or indeed, a Focus Fader V.1, which is just on/off, no curve. But I don't know if it fits.

well the 004 has a curve control but its like an on and off switch. Right side is sharper than the left side. So would a focus fader would work?

If the PCV faders work then so will the P&G fader. Much better, although i've heard the latest makes were absolute shit, something with production being moved.