Four million of anything is a lot, but for a small indie iPhone developer to rack up four million downloads of its games and apps (including in-app purchases) in two years is impressive. 

But that's the situation at US codeshop NimbleBit, which reached the magical number thanks in part to the release of its free Dizzypad HD for iPad, which was the number 1 free iPad game in the US last week.

We caught up with co-founder and programmer Ian Marsh to find out some more.

Pocket Gamer: How surprised are you to hit four million iPhone downloads?

Ian Marsh: I'd say we are happily surprised, but we've known from the beginning that the App Store is a market of extremes where one app can get a million or more downloads while the next flounders at a couple thousand.

Our very first app Hanoi, managed to climb to the #1 free spot in a similar fashion to Dizzypad HD in the early App Store days nearly two years ago. With more than 10 games on the store we've learned to leverage our hits to help our titles which have lower exposure.

How surprised are you that a large number of downloads doesn't necessarily result in a lot of revenue?

Obviously the majority of the four million downloads are for free apps, but the remainder allows us to stay very comfortably profitable and gives us the freedom to develop new ideas.

A free download can have a significant amount of value to a developer, whether it contributes to word of mouth, increases your brand awareness, gives ad impressions, or becomes a potential in-app purchase (IAP) customer.

How have things changed in 2010?

IAP has certainly been the biggest shift for us. Now that we have the ability to get revenue from free apps (without resorting to advertising) our goal is to get our games in the hands of as many people as we can.

[Note: Ian tweeted that IAPs make up around 60% of NimbleBit's revenues, with the company having now done almost 150k IAP sales.]

If done correctly, free + IAP games can serve the entire gamut of the App Store audience - the majority of users who don't want to pay for much, and the users content to spend as much as they think an app is worth.

If there is one thing we've learned over the last six months it is that the first priority should be getting as many people playing your game as possible; true fans will support your game when given an opportunity.

What do you think is the best strategy to encourage people to buy IAPs?

I think the best strategy is simply to offer more of what people are enjoying from the base free version. It is important to make the free version stand on its own to keep the non-payers happy fans.

The longer they continue to enjoy the free content the higher your chance of converting them to a paying customer (or the higher your chance they will spread the game through word of mouth).

What's your approach in terms of future apps?

Our approach is to continue to develop appealing free titles with optional IAP. Advertising is something we've never been a huge fan of, and would rather stick to IAP as long as it is continues to perform.

The reason many users can't believe some of our games are free is because there aren't ads cluttering up the game. Our number one goal is to continue to grow our library and user base until NimbleBit is a household name for iDevice owners.

How about supporting other mobile platforms?

Coming from traditional cell phone game development we wholeheartedly appreciate the simplicity of developing for the 75 million or more similar devices that Apple has managed to sell.

Being our only full-time programmer, I certainly don't have time to support 5 or more android ports of any of our games. With the number of Apple devices growing (iPad and the next iPhone) we think it's smarter to focus our energies on keeping our games on all of Apple's mobile devices.

Thanks to Ian for his time.

You can keep up-to-date, or indeed chat in real-time, to NimbleBit via its website.