https://github.com/googlei18n/glyphsLib/blob/cfef8fb7d189c3e52ba89b6c6ddaa99a1e85ff8d/Lib/glyphsLib/classes.py#L243
parse_hint_target() should accept indexes with 2, 3 and 4 values. {1, 2}, {1, 2, 3}, {1, 2, 3, 4}. That happens with TrueType hints that are attached to interesting segments.