Mahamud ibn Hamza (also known as Burhan ad-Din) al-Karmani محمود بن حمزة الكرماني wrote in his book asrar at-Tikrar fil Qur'an أسرار التكرار في القرآن (the deeper meanings of repetitions in the Qur'an) also known as al-Burhan fi twajih mutashabih al-Qur'an lima fihi min al-Hujjah wal Bayan البرهان في توجيه متشابه القرآن لما فيه من الحجة والبيان when discussing repetitions in surat an-Nisa' the following
73 - قوله : إن الله لا يغفر أن يشرك به . ختم الآية مرة بقوله : فقد افترى ، ومرة بقوله : فقد ضل ؛ لأن الأول نزل في اليهود ، وهم الذين افتروا على الله ما ليس في كتابهم ، والثاني نزل في الكفار ولم يكن لهم كتاب ، فكان ضلالهم أشد . (source)
My own translation take it carefully:
In the verses "Indeed, Allah does not forgive association with Him ..." the ending once was " has certainly fabricated a tremendous sin" and another time was "has certainly gone far astray" because the first was revealed (addressing) about the Jews and they have fabricated about Allah what is not in their (original) books while the second was addressing the disbelievers and they didn't have a book and therefore their misguidance was even worse.
This statement was basically adapted by imam as-Suyuti in his al-Itqan fi uloomi al-Qur'an (even if he warned others from some al-Karmani's books in the same book). A similar statement can be found in Khasf al-Ma'ani fi al-Mutashabih wa al-Mathani كشف المعاني في المتشابه من المثاني of the shafi'i scholar Badr ad-Din ibn Jamah بدر الدين بن جماعة
In his at-Tahrir wa Tanwir and when discussing verse (4:116) Ibn Ashur said (My own translation -with comments- take it carefully):
وإنما قال في السابقة فقد افترى إثما عظيما لأن المخاطب فيها أهل الكتاب بقوله يا أيها الذين أوتوا الكتاب آمنوا بما نزلنا مصدقا لما معكم فنبهوا على أن الشرك من قبيل الافتراء تحذيرا لهم من الافتراء وتفظيعا لجنسه .
Allah said in the former verse (referring to 4:48) "has certainly fabricated a tremendous sin" because the addressed audience were ahl al-Kitaab by " you who were given the Scripture, believe in what We have sent down [to Muhammad], confirming that which is with you ... (4:47)" where they have been warned from committing shirk as this is among the fabrication or invention as warning for them and to emphasize and frighten them from committing it.
وأما في هذه الآية فالكلام موجه إلى المسلمين فنبهوا على أن الشرك من الضلال تحذيرا لهم من مشاقة الرسول وأحوال المنافقين ، فإنها من جنس الضلال . وأكد الخبر هنا بحرف " قد " اهتماما به لأن المواجه بالكلام هنا المؤمنون ، وهم لا يشكون في تحقق ذلك .
While in this verse the speech is addressing the Muslims who were warned from shirk which is set equal to misguidance and as a warning from opposing the Messenger (read for example 4:115 and the verses before) and telling them about the hypocrites as they are part of those who have gone astray. And the word "قد" (translated as indeed/certainly) emphasises the importance of this topic as the addressed audience are the believers and they don't doubt that this would be true.
So basically the difference in the ending of these two verses comes from the audience addressed by these verses which can clearly be concluded from the context.