What can you do with a 6gb data plan for an iphone 4?

You can test it. Go to Settings > General > Usage > Cellular Usage > Cellular Network Data.

First select Reset Statistics so the numbers there will clear out. And then track how many data you consume in one day, one week while using your data.

Also, make sure your Telco doesn't charge you extra for Personal Hotspot because other networks do. 🙂

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It really depends on what you do. Without tethering, it would be hard to reach 6gb a month simply browsing safari and downloading/updating apps. Streaming a lot would definitely increase your usage. If you use your phone as a hotspot it'll be much easier to reach your 6gb limit.