Quran 4:104 was revealed just after the Battle of Uhud. The commentators on the Quran, such as Tafsir al-Jalalayn and Tanwir al-Miqbas min Tafsir Ibn ‘Abbas say, that the verse was directed specifically at the hostile Quraysh.
When Muslims were persecuted for thirteen long years in Makkah, the small minority fled to Madinah for safety. When they settled, the Muslims thought that their persecution which the Quraysh subjected the Muslim to, will no longer happen in Madinah, as they were 100s miles away. This was not long lived. The Quraysh once again were inciting tribes in Arabia to kill Muslims as a result of them abandoning their former faith ‘paganism’ for Islam. (Read how Quraysh continued persecuting Muslims in Madinah, before battle of Badr: ‘Did Quraysh Persecute Muslims When They Fled To Madinah?‘).
With the continued, aggression and hostilities of the Makkans, the Muslims were commanded to fight the Quraysh. This in turn led to the battle of Badr, and then the Battle of Uhud.
When the Quraysh lost at the battle of Badr, they demanded that they want to fight the Muslims. So not long after the battle of Uhud took place (Read more about the battle of Uhud here: ‘Battle Of Uhud‘). The Muslims lost and had heavy causalities on their side. After the Quraysh won in this battle, as they were going back, when this current verse (Q. 4:104) was revealed ordering the Muslims to go after those Quraysh members whom murdered Muslims.
“And do not weaken in pursuit of the enemy. If you should be suffering – so are they suffering as you are suffering, but you expect from Allah that which they expect not. And Allah is ever Knowing and Wise.” – Quran 4:104
As explained, the current verse (Q. 4:101) speaks of those Muslims who were wounded in the battle of Uhud. That they should go out in pursuit of those hostile enemies who killed many Muslims in the battle, 1400 years ago.
the Quraysh tormented, tortured and murdered many innocent Muslims. So those who were pursued weren’t innocent. They were guilty of many heinous crimes.
“After they returned from Uhud, the Prophet (s) dispatched a group to seek out Abu Sufyan and his companions, but they complained about their wounds, and the following was revealed: Be not faint, [be not] weak, in seeking, in pursuing, the enemy, the disbelievers, in order to fight them; if you are suffering, [if] you have pains from a wound, they are also suffering as you are suffering, that is, just like you, yet they do not shrink from fighting you; and you hope from God, in the way of victory and the reward for it, that for which they cannot hope, and since you have this advantage over them, you should be more willing for it than them. God is ever Knower, of all things, Wise, in His actions.” 
Tanwir al-Miqbas min Tafsir Ibn ‘Abbas:
“Allah then encouraged the believers to go in pursuit of Abu Sufyan and his men after the Day of Uhud, saying: (Relent not) do not slacken or weaken (in pursuit of the enemy) Abu Sufyan and his men. (If ye are suffering) from your wounds, (lo! they suffer) from their wounds (even as ye suffer and ye hope from Allah) His reward and fear His torment (that for which they cannot hope. Allah is ever Knower) of your wounds, (Wise) He enjoined upon you to go in pursuit of these people.” 
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 Tafsir al-Jalalayn http://altafsir.com/Tafasir.asp?tMadhNo=0&tTafsirNo=74&tSoraNo=4&tAyahNo=104&tDisplay=yes&UserProfile=0&LanguageId=2
 Tanwir al-Miqbas min Tafsir Ibn ‘Abbas http://altafsir.com/Tafasir.asp?tMadhNo=0&tTafsirNo=73&tSoraNo=4&tAyahNo=104&tDisplay=yes&UserProfile=0&LanguageId=2