POpen4 is copyright John-Mason P. Shackelford (john-mason@shackelford.org) and made available under the terms of Ruby's license, included below for your convenience.
Ruby is copyrighted free software by Yukihiro Matsumoto <matz@netlab.jp>. You can redistribute it and/or modify it under either the terms of the GPL (see the file GPL), or the conditions below:
1. You may make and give away verbatim copies of the source form of the
software without restriction, provided that you duplicate all of the
original copyright notices and associated disclaimers.
2. You may modify your copy of the software in any way, provided that
you do at least ONE of the following:
a) place your modifications in the Public Domain or otherwise
make them Freely Available, such as by posting said
modifications to Usenet or an equivalent medium, or by allowing
the author to include your modifications in the software.
b) use the modified software only within your corporation or
organization.
c) give non-standard binaries non-standard names, with
instructions on where to get the original software distribution.
d) make other distribution arrangements with the author.
3. You may distribute the software in object code or binary form,
provided that you do at least ONE of the following:
a) distribute the binaries and library files of the software,
together with instructions (in the manual page or equivalent)
on where to get the original distribution.
b) accompany the distribution with the machine-readable source of
the software.
c) give non-standard binaries non-standard names, with
instructions on where to get the original software distribution.
d) make other distribution arrangements with the author.
4. You may modify and include the part of the software into any other
software (possibly commercial). But some files in the distribution
are not written by the author, so that they are not under these terms.
For the list of those files and their copying conditions, see the
file LEGAL.
5. The scripts and library files supplied as input to or produced as
output from the software do not automatically fall under the
copyright of the software, but belong to whomever generated them,
and may be sold commercially, and may be aggregated with this
software.
6. THIS SOFTWARE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR
IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
PURPOSE.