If you have used your 4 GB pen drive to burn a bootable image for operating system and later that pendrive is showing as 970 MB instead of 4GB, follow below steps to recover it back to its original size
1. Right click on your computer > Manage> Storage> Disk Management
See how much Gb is shown for your pen drive.
It might be showing space in pendrive as 970MB and remaining as unallocated space.
Right click on unallocated space, if you get format option, you can format that and merge with the other volume to get total 4 GB
If you are not getting , go the software which you used to burn bootable image into the pendrive
Eg: poweriso >tools> cleanup USB pendrive
once that is done, you will get 4GB back