@julieb Thanks for reaching out!
You got some really good advice from @TomV in his response, so we'll focus a little bit more on the differences between the bikes you listed. Because you mentioned climbing hills as something you are concerned with, we'll talk about gearing ratios a little bit first. If you're interested in reading more, we recommend taking a look at the Expert Advice article How to Use Bike Gears.
As TomV mentioned, the more teeth you have in the back, the easier it is to pedal uphill. The Cannondale Trail Tango 3 has 9 gears in the back, with the largest having 36 teeth. The Cannondale Trail Tango 4 has 8 gears in the back, with the largest having 34 teeth. Effectively, what that means for you is that the Trail Tango 3 has an additional gear in the back that makes it easier to climb hills.
When you go up in a model of a bike, typically the components (everything on the bike that isn't the frame) are the driving factors of the price difference. Often times that difference can be weight, responsiveness, and speed. In these two bikes, the biggest difference is going to be the gear ratio and that additional, bigger gear, on the Trail 3. Both of these bikes have hydraulic lockouts for the front shock, which can be beneficial in climbing as it helps with efficiency. Both derailleurs (front and back) are the same on these bike models so shifting speed and function will be the same as well.
Hopefully this helps, please let us know if you have any other questions!