Battery charging time
Batteries are usually rated in Ampere hours (Ah), which is Amps x hours. So 1Ah could be 1Amp for 1 hour, or 2A for 0.5 hours, or 0.5A for 2 hours etc.

If you have a 1.5Ah battery and charge it at 1A then it should take 1.5 hours to fully charge from empty. However, depending on the battery's chemistry it may have to be charged at a lower rate, and current may have to be reduced once the voltage rises to its maximum permitted value.

Before deciding how to charge your battery you need to know what type of chemistry it uses. This may not be written on it, but you can get a clue from the rated voltage and physical shape. 4V suggests a 2 cell Lead Acid battery, because Lead-acid chemistry develops 2V per cell. However this an unusual configuration (6V and 12V are much more common), so it might be something else such as a single Li-ion cell (which is 4.1~4.2V fully charged). 

If you charge a battery incorrectly it may catch fire or explode, so it is vitally important to know what its charging requirements are. If you aren't sure then don't do it!