technically, you're supposed to use one after each use of the four track, or every couple of uses, depending on how much tape you ran over the heads during a session. i'm the only person i know that ever used a demagnetizer on his four track. i never noticed a sonic difference after using one but did it as a pre maintenance thing to keep my four track in good shape. i was also pretty good about keeping the rollers and stuff clean by using cleaner and rubber conditioner made for that purpose. I'm not sure which demagnetizer I had but it looked just like the Tascam E-3.
This looks like about what I did to the 4 tracks that I owned: