Hellos Shahriar21,
Welcome to the community!
Firstly, you queried about keeping your pre-paid account active. Are you wishing to be able to use services while away or merely keeping your phone number?
While the various top-up amounts have varying expiry dates, you should note that Fido no longer has a purely pay-as-you-go pre-paid option. If you wish to be able to use your services while abroad, you would need to pay the monthly plan fee. If your travels include the US, you should note that they have shut-down their '3G' networks and pre-paid US roaming is no longer an option (see here).
On the other hand, if you do not want to use your services abroad but only wish to keep your number, you wouldn't necessarily need to top-up. Since you are away for only 4 months, your number should remain even if you do not have a balance on your account (see here). That said, you might consider verifying with customer service. They have a dedicated pre-paid line (see here).
With regards to your wife switching to post-paid, I understand wanting to avoid set-up fees. However, you should note that there will likely be a migration fee for the change in services. I believe the fee to migrate services is $10, though it may have increased since this post. While that post refers to migration from post- to pre-paid, it would be the same for pre- to post-paid as well (see here). In order to migrate her services, she would need to contact customer service. In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above.
Hope this helps 😀
Cheers
