Can you charge a12V 4Ah with a 12V 5Ah with jumper cables?

Yes, you can use jumper cables to charge a 12V 4Ah battery with a 12V 5Ah battery, provided the 5Ah battery is fully charged. Connect the positive terminal of the charged battery to the positive terminal of the 4Ah battery, and the negative terminal of the charged battery to the negative terminal of the 4Ah battery. However, this method is not ideal for charging as it may not provide a controlled charging rate, which can potentially harm the batteries. It's better to use a proper battery charger for safe and effective charging.