Provided the 4230DAB's Garmin firmware is:
(i) not 'protected' against using unlocked maps (probably it's unprotected) and;
(ii) your downloaded map *.img is indeed properly unlocked (it will need to have been treated by gimgunlock - if required just google for it, version 0.04);
then the following will work:
  • Make a folder named Garmin in a CLEAN SD card previously freshly formatted in FAT32 only.
  • Place the unlocked map img file named only GMAPSUPP.IMG into the Garmin folder.
  • If you want to add safety camera warnings or other POIs as *gpi files then make a folder POI and add the GPI files (POI folder can either be in the card root or in the Garmin folder for most Garmin devices).
  • If it supports JunctionView, make a folder named JCV in the Garmin folder and place the appropriate *.jcv file for the map and your device in there. Google to check what additional files the 4230DAB supports).

To check your map is unlocked, open the IMG file in GMAPTool and look under 'Details' tab. If you see an "L" on any line under the heading 'prio' then it's not fully unlocked. You should see only "30" not "30 L S" for a City Navigator map.
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