Yes, it is generally safe to use a 3-pin Molex to 4-pin CPU adapter.
Yes, it is generally safe to use a 3-pin Molex to 4-pin CPU adapter.
You can safely connect the original 4-pin PSU to your setup. The adapter will bridge the molex-to-4-pin connection, allowing the B450 to work properly.
It isn't safe to use this setup. A 4-pin EPS connector can deliver up to 155 watts, while the 12v rail on a Molex connector supports about 53 watts—leaving a 100-watt shortfall. Also, avoid running your PSU; I’m not aware of any recent models with just a 4-pin design, and most should be replaced after 7–10 years based on their condition.