Leave extra wire in conduit for future expansion
There is no problem having unused conductors, properly capped, in your circuit. Since you'll total 8 wires you're within fill capacity for 3/4&quot; conduit (10 or 11 depending on type).
You might ask yourself whether a 60A or 100A sub-panel isn't a better strategy, though. Four aluminum wires might be cost-equivalent to the copper you're planning and you'd have many more future circuit options.