Well error number isn't an error number, it is a job number. Most likely it is one of several things
1. There are 3 versions of NFS, NFSV1, V2, and V3. Make sure everything is running same flavor.
2. Check permissions. Most likely the NFS server has a list of IPs that are allowed to mount, and .4 and .7 aren't on the list. If you are using DHCP then obviously don't hard-code IP numbers, use fully qualified names