Tools -> Options -> Languages-> All Languages -> Formatting
indent with tabs is checked, set indent to 4, tabs to 8, then apply; a confirmation box pops up asking if you are sure, select yes
go to Application Languages -> ANSI-C and chhose the same settings
then go back to All Languages formatting: The settings have been cleared out
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SlickEdit Pro 2016 (v21.0.0.9 64-bit)