You can use any type of RAM (Obviously the RAM needs to match the socket / pins to go in the laptop), although it is recommended to use the same type / brands / speed etc. If you don't, the RAM will run at the speed of the lower RAM stick.
Found the specs of your laptop (Useful if you post this sorta stuff in future if you have it to hand!)
Processor - Intel Pentium T4200 / 2 GHz ( Dual-Core )
RAM - 2 GB (installed) / 4 GB (max) - DDR2 SDRAM - 667 MHz ( 1 x 2 GB )
Hard Drive - 160 GB - Serial ATA-150 - 5400 rpm
Operating System - Microsoft Windows Vista Business / XP Professional downgrade
Screen - 14.1' TFT 1280 x 800 ( WXGA ) - 24-bit (16.7 million colours)
Optical Drive - DVD±RW (±R DL) / DVD-RAM - integrated
Graphics - Intel GMA 4500M
Bluetooth - No
Warranty - 1 year warranty
Double check that you do have another slot to utilise the dual channel, and use DDR2 SDRAM, and you should be fine..
Also just a point, you may have the 32bit version of Windows (Again this is only an assumption - you will be fine if you have the 64bit version), which may not show all 4GB of RAM even after it has been installed - but will still usually be utlising the vast majority of it.
Bare in mind, laptops are quite a bit more tricky to upgrade compared with PC's!