If you are increasing the Voltage from 48 Volts to 66 (?) Volts then a 48 Volt controller may or may not work with the new higher Voltage. The only way to know if it can handle the extra Voltage is to hook it up and see. Some controllers can handle the extra Voltage while other can not. There is no risk in trying the 48 Volt controller with a 66 Volt battery pack other than it burning out and needing to be replaced.
If a 48 Volt battery pack is replaced with a 60 Volt battery pack then a 60 Volt charger would be needed and should be used to recharge it. However if you are adding one new 12 Volt battery to a used 48 Volt battery pack then using a 48 Volt charger to recharge the original 48 Volt battery pack, and a 12 Volt charger to recharge the new 12 Volt battery would be best so the Voltages of the new and old batteries do not become unequalized. The 12 Volt charger would need to be an automatic type that shut down or goes into float mode after the charging cycle is complete. Some trickle chargers are automatic while others are not so make sure to get one that automatically shuts off after the battery is charged. Although a 12 Volt automotive wet cell trickle charger would work I have to recommend using a 12 Volt SLA battery charger instead because it is designed for SLA batteries and will recharge them to the Voltage level they are designed for.
If a 60 Volt battery pack will be recharged with a 60 Volt charger then it is recommended that all of the batteries in the pack be the same brand, age, and Ah rating. Otherwise the battery Voltages may become unbalanced which will damage the individual batteries in the pack and lower there lifespan.
Please let us know if you have any questions.
Yes the fifth battery would need to be wired in series with the other four batteries to make a 60 Volt battery pack.