sda4 is an extended partition---it acts as a "wrapper" for any "logical" partitions that you add. It appears that you created sda5 inside of sda4---and that you used all the space available. sda 5 is ~690 GB.
If you need 300GB, then use fdisk to delete sda5 and then create a new partition of 300GB. Beyond that, you'll need to tell us what you want to do---eg i assume you'll want to create a file system and then mount it somewhere.
Tell as much as you can about the context of all this, and we can give better help.