In a way it's true -- once you've recorded the track you can't put an effect onto that track in any way. What you can do is re-record the track after passing it through an effect and print a new effected track. That's the way it's always been with tape.
Anyway, in a nutshell here's the way you do it with a typical cassette multitracker. There is typically a set of jacks labeled Send and Return (often a single mono send and a pair of stereo returns.) There should also be sliders or knobs labeled Send for each track; if I recall there's probably only a single Return level control. Here's how it works: the channel Send taps off part of the signal recorded on that track or being played into that track. This signal is directed out the Send jack. An effects box (let's say a reverb) is hooked in to recieve a signal from the cable; its output is then patched into the Return jacks. You can then adjust the level of this signal mixed in with the direct levels on the channels themselves. By adjusting the send and return levels (and you can usually control a lot of this on the effects box as well), the amount of effected ("wet") signal is balanced against the amount of uneffected ("dry") signal, adjusted to taste by you, the mix engineer.
The limitations with cassette portastudios are usually that you can only apply one effect at a time. Fancier multitrackers and "real" mixers have individual sends and returns for each channel that can each be wired to a separate device. But with a cassette portastudio, if you want a chorus on a guitar track and a reverb on the vocal, you have to get creative.
One thing you can do is to record the track with the effect patched in the first place, but then you're stuck with the effect and cannot remove it.
You can also record the parts totally dry and add an effect while bouncing to another track. But with only four tracks you do not have the luxury of keeping the original dry track around in addition to copies of it with effects. You also lose some fidellity with each bounce. There's quite an art to it all, but skillful people can make pretty good recordings with these portastudios, working within the phsicla constraints imposed.
What's so neat about computer recording is that you can always keep the dry tracks and an almost unlimited number of alternative copies and keep them intact and distinct, and the effects can be added in the software to individual tracks or submixed groups of tracks... and when you do bounce tracks together, there is almost no loss of fidelity at all. And the fidelity you get right off the bat is so much better than a cassette multitracker is capable of.
