I'm not sure if it's doable, but this is what you can try:
1. Decrease the Minimal character count to trigger search option to 0 on the General Options -> Behavior panel
2. Turn on the Trigger search when changing a facet on settings? option on the same panel.
The only difference, that the categories can only be displayed in checkboxes. Other display types like radio, select box etc.. is coming in version 5.0. (or maybe sooner if I can finish)Best,
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