There are three main factors to consider when selecting a capacitor for replacement:
- Effective Series Resistance (ESR)
The capacitance is easy to match, and you probably want to match this as closely as possible, though capacitance can vary wildly from device to device, with environmental factors, and with time. I assume "MFD" here stands for "MicroFaraD" not MilliFaraD (that would be 5 Farad!), so 4700µF would be a reasonably close match.
For the voltage you need to have at least the same voltage rating. It doesn't usually matter if you go over, since it's a limit to the voltage it can handle. The voltage for a power supply capacitor should be above the maximum ripple voltage peak it has to handle. That can be higher than the voltage of the supply.
The ESR for a power supply is more critical. You need it to block as much DC current as possible while at the same time allowing as much of the 100 / 120Hz ripple noise to pass right through as possible. So you want the ESR at 100 / 120Hz (note, after full wave rectification the line frequency is doubled) to be nice and low.
The advantage of the big cans is that they give a large surface area for the capacitor plates, which in turn keeps the ESR down. Using a physically smaller capacitor may increase the ESR, but given the advances in technology and capacitor construction, that may well be somewhat offset.
So what's the worst that can happen if your capacitor isn't up to the job? Assuming you have the right voltage, all that you may notice is an increase to the background hum from the speakers. Of course, if the voltage of the capacitor is too low you may want to stand back as it explodes and fills the room with vaporised electrolyte and shredded paper and aluminium foil. That's always a laugh.