Brain fart , the voltage rating being higher is fine, it is just the current that causes it to blow. If 4A fast blow is working now, 8A fast blow should be fine as well and possibly a bit safer but you can't go wrong with using the same as what was in there before (unless it's a Singlepower).
Assuming the places where it's burnt are the leads to the power caps, it looks like they short circuited when you put the top back on and thus created arcy sparky. It ought to be fine especially if you say the thing still works but check for any other burn marks throughout the amp and make sure the capacitors aren't bulging or anything. And as a rule of thumb, try to minimize messing around with a circuit while it's on, it can be quite dangerous especially for power applications like an amp. I do it somewhat often but that doesn't mean it's safe, it just means I'm an idiot.
Edit: Also remember that caps store charge, especially ones that don't have some sort of bleeder resistor, so even when the amp is off you do not want to short the leads of a capacitor or more arcy sparky may happen.