I see you must have figured out where the slot was, and found that the 4GB card was enough space - at least for now.
But let me tell you that you MAY want to get a larger capacity card for future updates and data storage. I usually recommend at least an 8GB card, which should be large enough for the next couple of years. But these "smaller capacity" cards are becoming harder to find - at least at the local retail level. So as a reference, I have a link to Amazon where is did a search for 8GB microSD cards, where there are lots of good quality cards of that size available.
8GB microSD card SEARCH
Also, I wanted to send another link to Garmin's page that mentions the largest capacities and speeds for various units - including the various Nuvi series devices.
By the way, your unit SHOULD be able to handle up to a 32GB microSD (SDHC) and up to Class 10 (speed).
I hope this provides a little bit of more detail about these cards and what your device will most likely need for future updates.
Please let me know if you have any more questions about your device or the update process.
Take Care & Travel Safe, Greg A.
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