Enable NFS 4.1 on Synology

Enable SSH from Control Panel -> Terminal and SNMP
Connect using Putty, login with your Synology admin credentials
Enter the following commands:
cd /usr/syno/etc/rc.sysv
sudo vi S83nfsd.sh

Go down to line 90, position your cursor at the N on the line "/usr/sbin/nfsd $N"
Press "i" to begin editing
Change the line to:
/usr/sbin/nfsd $N -V 4.1
Press Esc to finish editing

Type the following to save:
:w

Type the following to quit:
:q

The following will restart the NFS service:
sudo ./S83nfsd.sh restart

Now, if you run the following command you should see NFS 4.1 enabled:
sudo cat /proc/fs/nfsd/versions