There are a total of 21 4-letter words containing 'io', and the number may vary in different dictionaries. You can specify the dictionary and order.

• agio ₆
• biog ₉
• bios ₇
• brio ₇
• cion ₈
• diol ₆
• ions ₅
• iora ₄
• iota ₄
• lion ₆
• olio ₅
• pion ₈
• riot ₄
• thio ₆
• trio ₄
• viol ₉
• gios ₅
• hioi ₇
• pioy ₉
• skio ₈
• whio ₁₀

There are 21 4-letter words containing io in the well-known word game dictionary, 15 in the Scrabble US (US/Canada/Thailand - TWL/NWL) dictionary, and 20 in the Scrabble UK (International - SOWPODS/CSW) dictionary, and 16 in the Words with Friends (WWF) dictionary.