I have been unsuccessful to play rendered clips using Quicktime, VLC media player (they partially start but don't complete, and also WMP 11 (it doesn't play at all.) Previously all worked when compile from LR4 directly. An MP4 file is created and should be playable.

Any ideas what the issue is. I have recently downloaded Premiere Elements, but haven't tried compiling it yet.

Am I missing something?

You should be able to play 1080p and 720p footage without problems up to quality "high" - the higher the resolution and quality, the more problems normal players might cause. When exporting from LR4 you never had quality settings as high as now and the 3K/4K footage refused to play on some players as well. This is more a problem of the players and the hardware you are using.
But as I said - output qualities over "high" and resolutions higher then 1080p are mainly for post processing, and they work very well in Video processing programs.

You should be able to play 1080p and 720p footage without problems up to quality "high" - the higher the resolution and quality, the more problems normal players might cause. When exporting from LR4 you never had quality settings as high as now and the 3K/4K footage refused to play on some players as well. This is more a problem of the players and the hardware you are using.
But as I said - output qualities over "high" and resolutions higher then 1080p are mainly for post processing, and they work very well in Video processing programs.

I am rendering 1080p high, it takes too long if I try anything higher.

I use a Dell XPS 1302 laptop with NVIDEA GEForce GT540M processor, with 4G graphics memory and 6GB RAM onto a Seagate4TB backup Pro.

I think there is a problem with Media Player, which is a pity, as I don't use the others much and they are not working well with everything running.

I will try loading the sequences into Premier Essential and see if that works. WMP plays them well on my work laptop, but possibly because nothing else with heavy graphics usage is running.

I don't want to change my Laptop till next year, but could upgrade the RAM to 16GB to help speed up my machine if you think that would help.

Thanks Ian

Just use a lower quality setting. It will be still good enough!