First, Page Plus is a Verizon MVNO, not Sprint.
Second, the iPhone 4 is a CDMA only, non-LTE/VoLTE capable handset, and it's going to stop working on Verizon here in a couple months anyway. You're going to lose your prepaid balance no matter what.
Third, there's cheaper options than Page Plus for Verizon service these days, such as Red Pocket. Annual PAYGO plans are pretty well dead with everyone but H2O Wireless now, and they're AT&T-based. There's dirt-cheap Sprint options, but given the T-Mo/Sprint merger, who knows how long that'll last, so best not to bank on it and deal with the headache of another migration. Plus, Sprint and Idaho aren't a great combo past the interstate. I mean, it's not as bad as Montana or Wyoming or the Dakotas, but you probably went with a Verizon MVNO for a reason... that reason's not really changed much.
Fourth, the iPhone4 doesn't get security updates anymore, and there's some serious unpatched vulnerabilities in iOS.
Things to consider before even broaching the issue, and definitely something you'd be better to get in front of than have it blindside you this January 1st (or sooner) when you wake up, grab your phone and find you have zero network coverage.
As for addressing spam callers, Google Voice is one way, but then you get Google's wishy-washy GV support, their datamining, and whatnot. More modern smartphone operating systems have ways to filter/block out unknown callers from ringing through without going to a third party, and there's decent alternatives like Hiya that doesn't involve porting your number out from your phone, though read their ToS.
https://support.apple.com/en-us/HT207099https://www.androidauthority.com/how-to-block-a-phone-number-246484/https://hiya.com/There's also the hack of just setting up custom ring call groups. Set the default ringer to a silent/empty audio file with no vibration, and your known contacts to your regular ringer. Then you won't even hear the phone unless it's someone in your address book, and/or they left a message.
Another thing to consider, porting your number to GV and forwarding to another cell number isn't going to solve the spam caller issue, because it'll keep happening with the new number, possibly worse than you have already. On-phone filtration is better than adding a second number and potentially multiplying the woes.
Multiple ways to skin this particular cat.
Edited to add a couple points of interest I forgot.