Does tone sandhi apply to 成语
Similar as for words, tone sandhi also apply for 成语. So 一年四季 is pronounced as "yì nián sìjì", 一波三折 is pronounced as “yì bō sān zhé” and 一马当先 is pronounced as “yì mǎ dāng xiān”. 一触即发 however is pronounced “yí chù jí fā”.