File sharing and synchronization service provider Dropbox this morning announced that it has passed the 4,000,000 user milestone. The startup, recently crowned the Best Internet Application at the Crunchies 2009 awards ceremony, has also announced that it has hired a VP from Salesforce.com to lead its marketing and sales efforts going forward.

Dropbox was founded by CEO Drew Houston and CTO Arash Ferdowsi in 2007, and received seed funding from Y Combinator soon after. The company went on to raise $7.2 million from Sequoia Capital, Accel Partners and Amidzad Partners.

Dropbox enables people to sync files and media across platforms and devices, in order to have them available from any location. The service also allows people to easily and quickly share files with others. Dropbox provides users with 2 GB of space for free (twice the storage Google offers), with add-on plans offering more storage and functionality for a fee.

The company is also announcing today that Adam Gross has joined the company as Senior Vice President, Marketing and Sales. Gross spent the past seven years at Salesforce.com as Vice President of Platform and Developer Marketing, where he helped grow the company from early-stage to over $1b in revenue. During his tenure, Gross launched Salesforce.com’s Web services API and the Force.com platform-as-a-service.