Well, you've basically figured out the speediest way of getting from the airport to Tsimshatsui. I'm not sure how familiar you are with the MTR network in Hong Kong, but note that switching from the Airport Express (AE) line to the regular commuter line at Central will involve quite a bit of walking. As for the pricing deal, since I never took the AE unless I'm flying, I'm not that certain about what their latest deal is. I believe some of their deals are actually customized to attract local people using the popular "Octopus" electronic stored-value pass system, which means they might not benefit you.
Apart from switching trains at Central, you can also get off the AE train at "Kowloon" station. They have a number of free shuttle bus routes from there that can take you to various hotels in the area, including the Peninsular (the hotel and the office tower is on the same site). When you're done with your business, you can take the next shuttle bus back to Kowloon station and the AE back to the airport. The catch is that the shuttle bus frequency is about once every 20 minutes, and it probably will stop at various hotels before arriving at your destination.
Another alternative is to take the Airport bus no. A21, which takes you all the way to Tsimshatsui. This is actually the most direct way of getting there, and cheaper than the AE too. The catch is that the bus runs almost the entire length of Nathan Road, the major north-south thoroughfare in Kowloon, where traffic can be horrendous at certain times of the day.
Anyway, 4 hours sounds quite sufficient for what you want to do.