I had a HAC4 and now have a PowerTap and the difference is huge. Do not use the HAC4 as any kind of power meter - it is simply too far off. Perhaps indoors with no wind or gradient it would be closer but in the real world it is way off. Keep in mind that the HAC4 only records once every 20 seconds so it often 'misses' gradients and does not take into account winds.
For example, I compared PowerTap recordings to HAC3 recordings over the same route/conditions and the readings averaged 30-50% off and sometimes even higher on climbs.
I liked the HAC4 and think it is a fine cycling computer but it is worthless as far as watts go. . .
Adam