your 100GB partition is /dev/sda2. This holds a LVM group and partition.
Show us the output of
you 400GB partition is /dev/sda4.
When we are lucky (since you said it's a new system) you do not have any data on sda4 yet.
What you want to do in this case:
- unmount sda4
- remove sda4 using fdisk
- enlarge sda2 using fdisk
- resize LVM volume group on sda2 using pvresize
- resize LVM partition and file system on sda2 using lvresize
If you already have data on sda4 you start decreasing sda4 and moving it to the end of the HDD using gparted.
Be warned: You need to have a backup of all data. Messing around with partitions and no experience has a great chance to end in a desaster. With experience it still has a great chance to end up in a huge fail.
If you search in the forum here you'll find more details about the single steps. If you have any issues let us know.