Saaiduna Abdullah Ibn Masud Radiallahu Anhu saw a person standing in prayer with his two feet together and judged it to be against sunnah. He advised the person that if he had practiced Murawah (leave a slight gap between the feet) it would be more preferable. (Sunan Nasai p.142 v.1)
With regards to your question, the spreading of the feet does not go against those ahadith, which describe the Companions as joining their feet with each other to form orderly rows. Furthermore, the spreading of the feet the distance equal to four fingers applies whether a person is reading by himself or in congregation.
The following is an excerpt from the Third edition of Fiqh al-Imam written by Shaykh Abdurrahman ibn Yusuf Mangera :
Abul Qasim Al Jadali reports ‘I heard Numan Ibn Bashir Radiallahu Anhu relate that the Prophet of Allah Sallallahu Alahi Wasalam faced the people and instructed: Straighten your rows. By Allah, you should straighten your rows or else Allah will create disagreement between your hearts.’ Numan Ibn Bashir Radiallahu Anhu then states ‘I saw each person join his shoulders with the next person and his knees and ankles with the next person.’ (Sunan Abi Dawud p.104 v.1)
There are a number of reasons why the joining of the feet with each other cannot be established from the above hadith:
The words of the hadith, which actually describe the joining of the feet, are not the words of the Prophet of Allah Sallallahu Alahi Wasalam but are the observations of the narrator, Numan Ibn Bashir Radiallahu Anhu. Furthermore, this type of observation is not even found in a majority of narrations that emphasize orderly rows hence it becomes clear that the Prophet of Allah Sallallahu Alahi Wasalam did not command the feet to be joined together.
The hadith tells us the practice of the Companions before the prayer had begun. In order to comply with the instruction of the Prophet of Allah Sallallahu Alahi Wasalam they attempted to form a straight line by joining their shoulders and feet together before the prayer commenced.
If for the sake of argument, we were to accept that the joining of the feet was maintained throughout the prayer, a number of questions would arise. For one, it would be enquired whether the feet should be joined together in all postures of the prayer or only during the standing posture [qiyam]? If the answer is that it is required only during the standing posture, then the question would be, What is the evidence for that? Why is this arrangement confined to the standing posture only and not required in any other posture? If the answer is that it is necessary in all postures of prayer, then the question would be, How will people in each row go about joining their feet and shoulders together whilst in prostration or in the sitting posture? As is clear, it would be quite impossible to achieve that. Moreover, if the counter-argument is that it is only necessary to have the feet together while standing up because it is difficult to do so in the other postures of prayer, then the answer to that would be, it is also very difficult, and sometimes quite inconvenient, for many people to ensure this joining arrangement during the standing posture as well, just as it is in other postures.
4. If it is considered necessary to join the shoulders and feet together on account of the above hadith, then why have the knees been excluded? As is mentioned in the above narration of Sunan Abi Dawud that the Companions joined their knees together, it should also be treated as an obligation throughout the prayer. However, one must be warned that standing even for a short while with one’s knees joined to the next persons can be quite painful, in fact impossible in some cases, when there is significant difference in the size of the two people standing next to each other.
5. Another interpretation of this hadith is that the narrator Nu‘man ibn Bashir only intended to describe how the Companions attempted to form extremely straight rows at the instructions of the Messenger, and not that they actually joined their feet, shoulders and ankles together. It is for this reason that the title of a chapter in Sahih Bukhari ‘Chapter on the Joining of the Shoulders and Feet Together Whilst Forming the Rows’ has been classified by Hafiz Ibn Hajar to be based on exaggeration. He writes in his commentary Fath al-Bari, [Imam Bukhari’s] reason for choosing this specific title is to exaggerate (mubalagha) the importance of straightening the rows and filling the gaps in between. (Fath al-Bari p.247 v.2) This means that the narration is not to be taken literally. Imam Shawkani writes in his book Nayl al-awtar: [The statement of the Companion] means, place the parts of the body [shoulders, etc.] in line with each other, so that the shoulder of each person performing prayer is in level with the shoulder of the next person. This way everyone’s shoulders, knees and feet will be in a single straight line (Nayl al Awtar p.65 v.3).
In clear words, this indicates that the real reason for joining the shoulders and other parts was to straighten the rows, not that the joining itself is an obligatory act.
6. Saaiduna Anas Radiallahu Anhu has further stated in a narration of Ma‘mar which Ibn Hajar has recorded in his Fath al-Bari,If I were to attempt this [joining the shoulders and feet together] with anybody today, they would scurry away like restive mules (Fath al-Bari p.247 v.2).
It is apparent from this statement, that even the Companions did not continue this practise after the demise of the Messenger. If it had been a continuous action of the Messenger [sunna mustamirra] the Companions would never have abandoned it, let alone speak of it in such a manner.
7. Once it is established that the primary reason for the Companions joining their feet together was to achieve perfect order in their rows, it can be easily understood that this joining of the feet is not required any longer, since in most of the masjids and places of worship today, the lines are well marked on the carpets, marble, and floor coverings. The worshippers by standing together with their heels on the markings will automatically come together in perfectly straight rows. There is no need to be so particular in joining the feet together to achieve this goal, let alone consider it the actual goal.