Assuming that md3 and md4 are contiguous on disks (fdisk -l to confirm):
- backup the 71MB of data in the /home filesystem
- umount /home and comment the line in /etc/fstab
- destroy /dev/md4 and comment /dev/md4 in /etc/mdadm.conf
- destroy 2 partitions used to build /dev/md4
- extend 2 partitions used to build /dev/md3 to the expected target size
- reboot system (should be needed to update device maps)
- extend /dev/md3 to match the new size
- extend the ext4 filesystem on / to match the new size
- create 2 partitions for /home
- build md device on top of those 2 partitions and update /etc/mdadm.conf
- format new md device
- mount it on /home and update /etc/fstab
It is classical stuff for experienced sysadmins, but it can be very painful if you are not confident enough with linux storage/partitions/filesystems