This will only change taxi advertising signs.

Download the latest SparkIV 
and make sure your GTA IV is able to accept mods (google for more info)
using "SparkIV" open "vehicles.img" located in :
C:\Program Files (x86)\Rockstar Games\Grand Theft Auto IV\pc\models\cdimages
then extract the following files from the archive and import 
them into "vehicles.img" using SparkIV

"taxi.wtd" & "taxi2.wtd"

rebuild and save 


For MODDED IV Taxi's

Using "SparkIV" open "vehicles.img" located in :
C:\Program Files (x86)\Rockstar Games\Grand Theft Auto IV\pc\models\cdimages

then find the taxi or taxi's you have modded and select that .wtd file then click "edit"
now select the file with the taxi adverts ie "taxi_signs_1" and then click import 
choose the extracted .png file from the "For Modded Taxis" folder in this archive,
click save and close 
rebuild and save