In order to plot an ECG waveform on the screen I imagine that you will need to take a sample every 50ms. This being the case you will need to sample at 20 samples/second for 288,000 samples (20S/s x 60sec x 60min x 4hours).
Given the above information I would reccomend the ADC40 or the ADC42
. Both of these units have a +/-5V input range and can sample faster than the required 20S/s. The ADC42 would be more adviable as it has a higher resolution (you can see more detail). The PicoLog software provided will be able to log the data for up to approximatley 1 million samples.