This might not be much help ! I have just started using LE4 and I am having all kinds of problems. The audio drops out at randon approx every 10 minutes. If I unplug the USB plug and re-insert it this usually solves the problem (at least for a further 10 minutes or so).
Now, today, on a new project, when I ask for a new Mono track I get Stereo. However, for an existing project, if I ask for Mono I get Mono.
Also with a guitar plugged in directly (via a Tascam US-144) and also a mic (via the Stereo inputs, as I cannot get Mono), I can hear the audio clearly through my headphones plugged into the US-144. However, there is no audio getting into Cubase - the mixer sliders show no audio.
I am VERY frustrated.