GRUB's stage 1 must be stored in the MBR (the first sector, not a partition),
GRUB's stage 1.5 still is before the first partition (that starts at sector
63) and GRUB's stage 2 is in /boot/grub, i.e., inside your partition number
So, you can only keep the partitions numbered 6, 7 (not actually required,
you could have a swap file or not swap at all) and 8 for a working Trisquel.
If you keep the partition number 4, I guess you want to enlarge it a little,
taking the space currently occupied by partitions number 1, 2 and 3.
Partition number 8 would take the space currently occupied by partition
All that is easily done from Trisquel's live system, which includes GParted .
An alternative is to backup all data on partitions 4 and 8 (you want backups
anyway), redo the install choosing this time to take the whole disk and, once
in the new system, restore the whole backup in the large (maybe 470 GB) /home
partition that was created.