Calls from Puerto Rico to mainland US numbers run approximately $1.00 per minute. At that rate, $4 covers about four minutes of talk time. That is a very short conversation, especially if there is anything meaningful to discuss.
Put more on the account. If the call limit at that facility is 15 minutes, the full conversation would cost around $15. For regular communication you want to have enough on the account to cover several calls before it runs dry. Loading $25 to $50 at a time is a more practical approach than small deposits that get eaten up in a single call.
Puerto Rico prison calls to US numbers are treated as international or long-distance calls even though Puerto Rico is a US territory, which is why the rate is higher than domestic state-to-state calls. There is not an InmateAid forwarding number solution that changes this the way it does for standard Securus or GTL calls, so the $1.00 per minute rate is what you are working with.