WARNING
Partitioning always holds a risk of data loss. Moving the left borders of partitions may lead to boot failure. Especially as you did a Wubi installation I can't predict how it will react.
MAKE A BACKUP BEFORE PROCEEDING!
First, you need to boot from a live system, e.g. your Ubuntu installation medium. From there, start GParted.
You can not ⁽*⁾ resize mounted partitions, so right-click on all partitions in GParted's list that show the grey lock symbol behind the first column and unmount them.
After that, enlarge the Extended Partition /dev/sda4
by moving its left border to occupy all unallocated space. If you don't understand why, read my very metaphorical and easy explanation here, please.
Now there is unallocated space inside the extended partition, which we can now attach to your Ubuntu partition /dev/sda5
. Move its left border to the beginning of the extended partition.
Tell GParted to apply the changes by clicking the green tick symbol.
Now all unpartitioned space should be covered. Task done, reboot and pray that you don't need your backup.
(*) Some file systems allow enlarging the partition to the right without unmounting, but shrinking or enlarging a partition to the left is never possible while mounted.