Google Ventures, an investment arm separate from the main company, has invested in Ngmoco, a studio and publisher with games on the iPhone and iPad, according to TechCrunch. The estimated amount: $3 million to $5 million.

This is part of a new, fourth round of funding that would value the company at above $100 million, according to the report. Ngmoco has been busy bringing in new investment this year, having closed a $25 million third round, announced at the end of February, for an unknown valuation.

Since it’s from the venture arm of Google, this funding looks a bit different from the search giant’s other recent social and gaming moves. It has invested heavily, reportedly paying up to $200 million for a stake in Zynga, $228 million for all of Slide (including a retention bonsu) $75 million for monetization company Jambool, and in April, an undisclosed amount for LabPixies.

The Ngmoco investment differs greatly in scale from all of Google’s most recent moves, resembling an ordinary investment rather than a strategic buy-in.

Still, there’s likely something more to the story than just testament to Ngmoco’s future. Like a few other mobile developers, Ngmoco has so far avoided Google’s Android mobile operating system in favor of Apple devices — apparently even telling Gamasutra in a recent profile that it would continue to design for the iOS only.

That’s too bad for Google, since Ngmoco is one of the few mobile developers that’s successfully creating free-to-play, social games, most notably We Rule and We Farm. It also has its own mobile social network for gamers, called Plus+, which is so far not available on Android. However, as the valuation suggests, the mobile social gaming business has yet to grow as big as the web-based form has.

More than just gaming, Google appears to be aiming for a credible social offering to rival Facebook. Support from Ngmoco could help, and an investment is a good way to ask for that support.