Install - partitioning problem ('unusable space')
You have too many partitions created. the count is: sda1 (primary), sda2 (primary), sda5-sda7 (logical partitions inside a extended partition), thats why sda4 comes after all the higher counts. A Master-Boot-Record-formatted disk only allows 4 partitions (primary or logical). You can use the space by resizing/moving partitions accordingly or eliminate sda4, resize the extended partition (which already contains sda5-sda7 (logical partitions)) to the complete remaining space and create your wanted (logical) partitions inside the extended one.