Is it possible to daisy chain multiple sensors to get more than four sensors operating off of one NXT brick?

Yes, you can. You will need to use a sensor multiplexer.

There are many ways to do that. But the simplest way is to just buy one. Here are all of the ones I know of:

HiTechnic Active

HiTechnic Passive

Mindsensors Passive

Mindsensor Active

That "mindsensors active" "multiplexer" is just a port splitter. It isn't a multiplexer of any sort.

thanks for the info.

When using the Mindsensors splitter, you also need to be mindful of the fact that you cannot attach sensors that consume too much power together. There is a limit to how much you can draw form each sensor port. Another important thing to note is that the splitter does not allow 2 or more devices with the same I2C addresses to work together. You also need to use sensors that either disable their internal pull-up resistors or that can handle the fact that there's multiple sensors on the bus that have them.
The HiTechnic MUX is very nice and allows you to have multiple sensors on there that are identical and have identical I2C addresses without issue. However, it is a "read-only" multiplexer. In other words, you cannot send commands to the connected sensor. For example, if you had a compass connected to the HT SMUX, you would't be able to send it the calibration command, but you could read the data from it just fine. The HT SMUX also allows you to connect both analogue -and- digital sensors, something the Mindsensors splitter does not, it only allows digital.

So, which one should you get? Depends on what you want to do

- Xander

Actually, the analog lines are connected, they are just all tied together (like all the other pins). So if you use I2C sensors that aren't reliant on the analog line, you can use the analog line with an analog sensor at the same time as you are using I2C sensors on the other ports.