Q: Is there NFS v. 3 support for Linux? NFS v. 4?
A: The "stock" Linux 2.2 kernels support NFS v. 2, only. (The 2.4 kernels incorporate standard v. 3 support.)
If running 2.2, you'll want Dave Higgen's patches providing an NFS v. 3 client, and H.J. Lu provides an NFS v. 3 version of the kernel-based NFS daemon. You should also make sure you have the latest nfs-utils package and an NFS v. 3 patch for the "mount" utility. For links to the necessary files, visit the Linux NFS Project site at http://nfs.sourceforge.net/ . You may also want to get a pre-patched version of "mount". .
Neither the NFS v. 3 client nor the daemon (yet) supports NFS over TCP; they are UDP-only. Nor does the server support the "async" flag, for reasons of data protection. Therefore, the NFS v. 3 code is often perceived as slower than the Linux-standard NFS v. 2 client and daemon code, in which async mode is the default (at a slight risk to data integrity).
NFS v. 4 client and daemon code is currently being developed by University of Michigan's Center for Information Technology Integration (CITI), and is currently (August 2000) in alpha-testing: http://www.citi.umich.edu/projects/nfsv4/ . It will support NFS over TCP, in compliance with the IETF NFS v.4 draft protocl specification: http://www.citi.umich.edu/projects/nfsv4/dist2/draft-ietf-nfsv4-02.html