No, Garmin Nuvi 250W is not compatible with SanDisk Extreme 4GB Secure Digital (SD) Class 10 cards.
According to the specs of your Garmin Nuvi 250W device on :
https://buy.garmin.com/shop/shop.do... it shows that it can only support SD cards on it.
SD cards are from 128MB to 2GB only and SDHC cards are from 4GB to 32GB in capacity. According to the specification, your device supports only SD cards. Please click on the link below from our knowledgebase which explains the difference between SD and SDHC cards:
http://kb.sandisk.com/app/answers/d...