I have a Lexar 4GB, once I committed formatting stick and now shows that there is only 142Mb and the remaining space is no longer available, once again when formatting has 126Mb and 142Mb used to get, which is very little. Is the stick is damaged or there is an option to go back to 4GB by default. greeting

SOURCE: memory stick proo duo corrupted

with this program you can
http://www.download3k.com/System-Utilities/Backup-Restore/Download-PSP-Memory-Card-Data-Recovery.html

SOURCE: Memory sticks empty and shows no memory available

i have a solution that worked! Notice: save all the needed data on your computer, because Formatting your Memory stick will wipe everything out!
if you have a Sony PSP, or know someone who does, replace the memory stick that says "no memory Space" in to the PSP port, go to "Settings" which is the left-most section of the PSP main screen, then scroll down to "System Settings," select it, Scroll down again to the "Format Memory Stick" selection and press "X" follow on-screen instructions and  SHAZAM!  its ready for action in your camera again! 
-Indy

SOURCE: how to format a sony 4gb memory stick pro duo

Why do you need to format. If using on Sony products its ready to go no need to format. If needing formatting your PC will tell you so and will give you the option to do so by selecting yes to format and will do it for you.... Do not format unless prompet to do so

SOURCE: i have a memory stck inserted i formatted it on my psp

that is it . there is no enough space for your demanded progress . that means you should buy more space next time . rate please !

SOURCE: I formated my TDK USB-STICK 4GB TRANS-IT flash

Reformat with this utility.

http://www.pcworld.com/downloads/file/fid,64963-order,4-page,1/download.html