Hello moehao,
can you answer these questions:
(1) What type is the 4.8V battery: lead acid (LA), nickel cadmium (NiCd), nickel metal hydride (NMH), Lithium Ion (LiIon)? The terminal voltage of 4.8V would imply a stack of four NiCd or NMH cells.
(2) What is the capacity of the battery (mA/Hours or A/Hours)?
(3) What is the maximum power output of the solar panel (Amps)?
To answer your question, in general you can either use a purpose designed off-the-shelf controller to do your job or possibly a switch mode power supply which, depending on your answers above, should be quite cheap and, in Singapore, should be freely available.
spec