John Hering, co-founder and chief executive of Lookout

Lookout Mobile Security, which specializes in armoring smartphones from hackers, said today that it's raised an additional $19.5 million in funding.

The San Francisco-based startup says it now has nearly 50 employees and about four million registered users of its software, which includes a spyware scanner, remote backups, and a stolen phone locator. That's up from a reported two million users in September and three million in November.

Lookout's security apps currently are available for Android, BlackBerry, and Windows Mobile. In an interview with CNET, Lookout CEO John Hering said an iPhone version will be "coming very shortly" and customers should expect to "see something in 2011."

New features in Apple's iOS 4 operating system, announced in April and made available a few months later, aid development, Hering said. Those changes "enable us to do quite a bit more," he said.

Some of Lookout's features, like remote wipe and and a more comprehensive remote backup, are available only to customers who purchase the premium version for $3 a month. (Here's our review of the Lookout Premium service from last month.)

Today's funding round came from Index Ventures and existing investors Accel Partners and Khosla Ventures.

CNET's Elinor Mills contributed to this report.