I am using Ubuntu 12.04 and kdenlive 0.8.2.1.

When I tried to render into avi, the 'render to file' button is shaded, parameters are not accessible, and I can not render.

So, I rendered in H.264 and went to the terminal to convert the mp4 file into avi. You know those colleagues ;)

Though the terminal converted it, I had the following warning:
ffmpeg version 0.8.1-4:0.8.1-0ubuntu1, Copyright (c) 2000-2011 the Libav developers built on Mar 22 2012 05:09:06 with gcc 4.6.3
This program is not developed anymore and is only provided for compatibility. Use avconv instead (see Changelog for the list of incompatible changes).

Could it be the reason kdenlive do not render in avi anymore on my system?

Thanks,
--
Pierre

What ffmpeg command line did you use when converting from the terminal?
You mention mpeg4 but did you use "-vcodec mpeg4" or did you use "-vcodec xvid" as that one isn't developed anymore and only part of ffmpeg for compatibility. The mpeg4 codec is nowadays of equal quality.

It's just this isn't it about avconv?

ffmpeg version 0.8.1-4:0.8.1-0ubuntu1, Copyright (c) 2000-2011 the Libav developers
built on Mar 22 2012 05:09:06 with gcc 4.6.3
This program is not developed anymore and is only provided for compatibility. Use avconv instead (see Changelog for the list of incompatible changes).
Hyper fast Audio and Video encoder
usage: ffmpeg [options] [[infile options] -i infile]... {[outfile options] outfile}...

Use -h to get full help or, even better, run 'man ffmpeg'

AVI is not a very good container for h264, better to use mp4 or mkv really unless you have specific software that doesn't support either.

mp4 to avi is a quick remux rather than transcode as they are both just containers. So -vcodec copy -acodec copy.

Where do you see render to avi? Did you just change the file extension in the Output File box. If avi is not an option for the codec you want and as long as avi is supported with your chosen codec then you probably need to create a render profile for example h264 in an avi, if that's what you want.

As extra info to yellow's answer:

Note that mpeg4 can be two different codecs being mpeg-4 ASP (h.263) and mpeg-4 AVC (h.264).

What is commonly known as xvid/divx/mpeg-4 is all mpeg-4 ASP or h.263 quantization, and it's all the same codec according the same standard! Like a zillion NLE video editors who all create the same DVD compliant stream (mpeg-2), but one does it slightly better, or faster and uses other option which are defined in the same standard.

So again: mpeg-4 ASP is completely different from mpeg-4 AVC (h.264 quantization).

xvid is no longer maintained and most possibly avcodec is by now (far?) more advanced then xvid (or divx, which is even older and proprietary). Some years ago xvid did have an advantage for creating better quality mpeg4 ASP at the same bitrate, but these days are gone.
(Note that early versions of xvid and divx also modified metadata or encoding which sometimes made them incompatible as programs or hardware players read the metadata and "didn't understand it", whereas the video stream was actually the same. Like Microsoft putting in another standard being 99,9% the same as another industry standard, but...).

ffmpeg is still being developed by the FFmpeg team, but you are using the Libav fork of it, which puts that misleading message into the legacy version of the ffmpeg utility that it provides.