What distro are you running? I run Fedora, all USB configs happen automatically & works perfectly.
I am unfamiliar with the term "pen drive". What kind of USB device is it? Flash card reader, memory stick, external hdd, etc.
You posted that "cat /proc/partitions" shows sda1,2,4 as existing. Do you have any scsi or sata devices installed? If not, what are these entries referring to? /dev/sda, sdb, etc. is first & foremost used for scsi devices. Linux usually uses the next /dev/sd? that is available for USB drives, sata drives & other non IDE data storage devices.
If you don't have any scsi devices, or you know that you have no other use for the sda mount-point dirs in /mnt, delete them all & reboot. Then plug-in your USB device & check /mnt for any new entries.