An-Nisa 4:17
إِنَّمَا التَّوْبَةُ عَلَى اللَّهِ لِلَّذِينَ يَعْمَلُونَ السُّوءَ بِجَهَالَةٍ ثُمَّ يَتُوبُونَ مِن قَرِيبٍ فَأُولَٰئِكَ يَتُوبُ اللَّهُ عَلَيْهِمْ ۗ وَكَانَ اللَّهُ عَلِيمًا حَكِيمًا
Pikthall translates:
Forgiveness is only incumbent on Allah toward those who do evil in ignorance (and) then turn quickly (in repentance) to Allah. These are they toward whom Allah relenteth. Allah is ever Knower, Wise.
where the term "only incumbent" lacks a direct link to the original.
Sahih International leaves the word "only" but leaves out the word "incumbrent":
The repentance accepted by Allah is only for those who do wrong in ignorance [or carelessness] and then repent soon after. It is those to whom Allah will turn in forgiveness, and Allah is ever Knowing and Wise.
The particle إِنَّمَا (inna-maa) is translated as "only". However, this would imply that repetence is not possible for any wrong that is not done in ignorance, whereas other passages imply that repetence from sin, even kufr, is possible through due repetence.
Itani leaves إِنَّمَا untranslated:
Repentance is available from God for those who commit evil out of ignorance, and then repent soon after. These—God will relent towards them. God is Knowing and Wise.
An overview on the usage of the particle إِنَّمَا in Sunna drawa a picture where the meaning "only" does not fit. Rather, the affirmative إِنَّ, (inna: "indeed", "truely", "verily") fits well in all cases. Keyzer translates thus into Dutch:
Waarlijk, bij God is verzoening voor hen, die in onwetendheid gezondigd en er spoedig berouw over hebben. God vergeeft hun; want God is alwetend en wijs. (Verily, with God is atonement for those who have sinned in ignorance and are quick to repent. God forgives them; for God is omniscient and wise.)
The Quran continues (4:18):
But there is no atonement for those who do evil until death strikes them, and then (after death) say, "I repent"; nor for those who die as disbelievers. We have determined a severe punishment for these.
Is it correct to assume Keyzer's translation that the Quran mentions the two extreme cases; in verse 17 where God promises that He will for sure be forgive, and the case of a notorious evil-doer who did not repent or refrain from doing evil during his entire life?
Suggestion taking into account the answer of UmH:
Repentance is indeed available from God for those who commit evil in carelessness and repent in time. God will relent towards them. God is Knowing and Wise.
But there is no atonement for those who do evil until death strikes them, and then (after death) say, "I repent"; nor for those who die as disbelievers. We have determined a severe punishment for these.
This would fill the gap between the two examples, and the question, whether it is appropriate to add "only" in (17) becomes less relevant.