Partition /dev/sda4 is basically a holder for all the space allocated for an extended partition. Partitions sd5..sdn are allocated from this space. This was implemented a long time ago so that larger disks with more than four partitions could be accomodated. Thus your problem is to expand the expanded partition pool (sda4) and then you can expand the subpartitions. Since you are including an area ABOVE you'll be unable to save the partition data in the subpartitions.
Your question is YES you can expanded the extended partition, using the following process:
0. remove /dev/sda5 and /dev/sda6 from /etc/fstab and reboot system
1. delete /dev/sda6
2. delete /dev/sda5 (yes you will lose the filesystetm and information so backup)
3. delete /dev/sda4
4. Recreate /dev/sda4 with all the available disk including the currently unused space.
5. recreate /dev/sda5 with the size that you want
6. recreate /dev/sda6 with the size that you want and mark as swap
7. reboot the system
8. do mkswap on /dev/sda6
9. mkfs -t ext3 on /dev/sda5
10. mount /dev/sda5
11. restore backup onto /dev/sda5
11a. return /etc/fstab to include /dev/sda5 and /dev/sda6
12. reboot the system to see if all works
That is what you need to do.