Austin,... you have a problem!
First, it's not clear which Tascam 4-track you have.
Second, any undue humming internal to the recording process would be a bad sign, and is usually a job for a service tech.
Third, there's a large array of things that might cause hum on an external line signal, that feeds the recorder, f/i, if you have an overdriven electric guitar near a computer monitor, wall-wart power supplies near unshielded signal lines or cables, etc.
Try to make sure it's not an external interference coming in through your input source or cabling.
If it's just a hum, that's regardless of any external influence, then get back to my previous statement, it may be a service issue.
You may want to record a blank tape, with no line input at all, to see if the Tascam 4-tracker hums without any external sources.
Load a tape, unplug all mics and guitars/kybd's, and record a tape of silence, with faders up to the "7" mark, and any TRIM turned all the way DOWN, [toward "LINE"]. Make sure any obvious noise makers are off, or sufficiently far away from the Tascam. Also, make sure the Tascam is not sitting on top of any noise generating component, like a MiniDisc recorder or CDR recorder. Minidisc units, by my experience, will throw all sorts of RFI into the Tascam, if the Tascam's placed right on top of it.
There's a few other things that would cause internal hum to a Tascam 4-tracker. One more, is that if the power supply module is not matched properly to the 4-tracker, as in the case of the 424 and 414, both units require an external power supply.
Anyway, there's a few things that you may check out yourself, otherwise refer all servicing to a qualified tech. There may be a shorted internal contact, or some other problem that would requre a technician.
Based on your description, it's hard to tell, but those are some troubleshooting ideas for'ya.
