The Play Store page says that Android 4.1 is fine, so you should be good to go there.
I would strongly like point out that Fongo is not a replacement for your normal cell service, which they do not provide. What they do provide is a way for you to make calls for free. You will still need some sort of internet, being either wifi or through a cell service.
The app is free, there are no activation fees nor monthly fees for the free services. The free services include incoming and outgoing calls to most of Canada, and incoming texts.
There are couple of text plans starting from $2.79 a month up to $9.99 for 6 months. Both are unlimited.
To call outside of Canada you load your account with world credits and they get deducted as you make calls. You do not require credits to call within Canada.
If you want to move your number to Fongo, then there is a one time $25 cost for this.
What I suggest to anyone who is new to Fongo is to download their app and test it out with the free number they give you. All it will cost you is a bit of your time.
From there you can decide if Fongo and the services they provide is right for you.
More information and download links can be found here
https://www.fongo.com/fongomobile/
If you have other questions, just ask. There is usually someone around who can answer them.