There is an way to turn the cars into driveable ones.

FIRST STEP.
Spawn any vehicle from the game (WARNING, DO NOT DO IT WITH HEAVY VEHICLES LIKE AIRPLANES AND BOATS.)

STEP 2. Spawn in Jalopy or Jeep

STEP 3. Nocollide both.

STEP 4.. Get the GTA 4 Car on the jeep ( Put it inside the jeep)

STEP 5. Weld them both.

There it is, your driveable GTA IV car, No Vlad included.

Video Tutorial Coming Soon.