What is the detailed analysis of the fourth opinion which states that the pregnant or nursing woman does not need to make up the days and nor does she need to pay a fidya?

This opinion, which is that of Imam ibn Hazm, states that in the following hadith, a special allowance was given to the traveler and the pregnant or nursing woman, in which the obligation of fasting was dropped and was not replaced with any other act of worship:

Narrated by Abu Dawood, al-Tirmidhi, al-Nasaa’i, and ibn Maajah:
The Prophet (pbuh) said: “Allah has relieved the traveler of half the prayer and of the duty to fast, and He has relieved pregnant and nursing mothers (of the duty to fast).”
Classed as saheeh by al-Albaani in Saheeh abi Dawood

The rebuttal from the majority of the scholars is that this hadith needs to be combined with the verses on fasting in the Qur’an in order to make the ruling. It cannot be taken on its own, extracted from this hadith alone and isolated from other evidence, to build a major ruling like this.