Mim, ba, and waw involve the lips, while fa involves the upper front teeth and lower lip. Their phonetic proximity tempts some readers to weaken still mim before fa or waw. This is an error: the rule remains labial clarity (izhar), not labial concealment.
«هُمْ فِيهَا»
They are in it. Make mim clear before fa.
«عَلَيْهِمْ وَلَا»
Upon them, nor. Keep mim distinct before waw.
Do not overhold the lip closure either, as this inserts an artificial gap. Proper izhar allows mim and the following consonant to remain individually audible even when the passage is recited fluently.