Two different fuses is the correct answer. If you study the schematic you will see that the mains wiring goes from the upper pin on the power connector (the live pin when using a UK plug) to the single pole switch then through the T8AL fuse, marked 115V, to the top of the upper transformer primary and the voltage selector. So the 8A fuse is always in circuit, regardless of the voltage setting. If the voltage selector is set to 115V, as in the schematic, then the T4AL fuse, marked 250V, is left hanging at one end and so is not in circuit. For 115V operation, therefore, the 8A fuse is relied upon for protection. If the voltage selector is set to 230V, as shown in dotted lines in the schematic, then the T4AL fuse is connected in series with transformer primaries and the TA8L fuse. Since the 4A fuse has a lower current rating than the 8A fuse, it will (at least in theory, and it usually does) be the only fuse to blow in the event of a current overload. So for 230V operation, only the 4A fuse is relied upon for protection and the 8A fuse merely says 'ouch, that hurt' when things go boom. This arrangement is just a sneaky way to simplify the wiring and reduce manufacturing costs. Hope it helps.
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