My SD card is 4gb now its showing 30mb and I can't format

You have to be the computers Administrator to change any values on any Hard Drive on your computer. Click start control panel user accounts on the users tab click the name of the computer Administrator you must have full control. Click start click to open on my computer you should see your Usb drive E: or something right click select properties there will be an array of options select the sharing tab option then advanced sharing make sure you have a tick in the share this folder box then click permissions. Make sure all 3 boxes are ticked full control change and read in windows 7. Click start control panel user accounts click on the administrators account if you have made one or you will have to create one in windows XP. Click start right click on my computer select open
Right click the icon of your Usb Drive (It should be E:/ or something)
Click Properties in the menu that comes up
Click the Sharing tab
Click permissions
And finally, click Allow next to Full Control.

Click start control panel administration tools computer management disk management right click on your usb drive select properties click tools you should see click check now click start two boxes Automatically fix files and scan for and attempt recovery of bad sectors place then a tick in one box then select check now start or you might tick one box then return after its finished the first then return and tick the second box you should not do anything while the chkdsk utility is in progress any input may damage this drive Click start click on my computer to open you should see your drive right click then select format another way Click start control panel administrative tools computer management disk management Right click on your usb drive select format.
Hope this helps.