In plain words
The immature, not the wise, speak of Sāṅkhya and yoga as different. One who is properly established in either gains the fruit of both.
Word for word
The childish, not the wise, declare Sāṅkhya and yoga [to be] separate; one who has properly resorted to even one gains the fruit of both.
The commentators
Bhagavadgītābhāṣyam
संन्यासकर्मयोगयोः भिन्नपुरुषानुष्ठेययोः विरुद्धयोः फलेऽपि विरोधो युक्तः, न तु उभयोः निःश्रेयसकरत्वमेव इति प्राप्ते इदम् उच्यते —
साङ्ख्ययोगौ पृथक् विरुद्धभिन्नफलौ बालाः प्रवदन्ति न पण्डिताः । पण्डितास्तु ज्ञानिन एकं फलम् अविरुद्धम् इच्छन्ति । कथम् ? एकमपि साङ्ख्ययोगयोः सम्यक् आस्थितः सम्यगनुष्ठितवान् इत्यर्थः, उभयोः विन्दते फलम् । उभयोः तदेव हि निःश्रेयसं फलम् ; अतः न फले विरोधः अस्ति ॥
ननु संन्यासकर्मयोगशब्देन प्रस्तुत्य साङ्ख्ययोगयोः फलैकत्वं कथम् इह अप्रकृतं ब्रवीति ? नैष दोषः — यद्यपि अर्जुनेन संन्यासं कर्मयोगं च केवलम् अभिप्रेत्य प्रश्नः कृतः, भगवांस्तु तदपरित्यागेनैव स्वाभिप्रेतं च विशेषं संयोज्य शब्दान्तरवाच्यतया प्रतिवचनं ददौ ‘साङ्ख्ययोगौ’ इति । तौ एव संन्यासकर्मयोगौ ज्ञानतदुपायसमबुद्धित्वादिसंयुक्तौ साङ्ख्ययोगशब्दवाच्यौ इति भगवतो मतम् । अतः न अप्रकृतप्रक्रियेति ॥ ४ ॥
In plain words
One might think it follows that renunciation and the path of action, being carried out by different kinds of people and opposed to one another, should also be opposed in their result — and not that both equally lead to the highest good. To answer that, this is now said —
'Sāṅkhya and yoga, kept apart, with different, conflicting results' — that's what the ignorant say, never the wise. The wise, the ones who truly know, hold that both lead to a single, undivided result. How is that? Whoever properly commits to even just one of Sāṅkhya or yoga — practicing it fully and correctly — obtains the fruit of both. Because the fruit of both is one and the same highest good; so there is no real conflict in what they lead to.
Objection: he set up the topic with the words 'renunciation' and 'the path of action' — so why does he suddenly talk about Sāṅkhya and yoga having one fruit, which is beside the point? Reply: this isn't actually a problem. Even though Arjuna's question was framed purely in terms of 'renunciation' and 'the path of action', the Blessed Lord, without dropping those terms, folded in a distinction he himself intended and answered under a different name: 'Sāṅkhya and yoga.' It is his own view that renunciation and the path of action, when joined with knowledge and an even-minded approach to its means, are exactly what the words 'Sāṅkhya' and 'yoga' refer to. So this is not off-topic at all.
Close to the Sanskrit
When it might seem established that, for renunciation and the path of action, being performed by different persons and opposed to each other, opposition would logically hold even in their fruit — but not that both alike bring the highest good — this is now said —
'Sāṅkhya and yoga, as separate, with opposed, distinct fruits' — this is what the childish say, not the wise. But the wise, the knowers, desire a single fruit, not opposed [for the two]. How so? One who has properly adhered to even one of Sāṅkhya and yoga — meaning has properly practiced it — obtains the fruit of both. For the fruit of both is that very same highest good; therefore there is no opposition in the fruit.
Objection: having introduced the topic using the words 'renunciation' and 'the path of action', why does he speak here of the oneness of fruit of Sāṅkhya and yoga, which is off the topic? Reply: this is no fault. Although Arjuna's question was framed with only 'renunciation' and 'the path of action' in mind, the Blessed Lord, without abandoning those very words, but adding a distinction of his own intention, gave his reply under a different designation — 'Sāṅkhya and yoga.' It is the Blessed Lord's own view that renunciation and the path of action themselves, endowed with knowledge and an even-mindedness regarding its means, and the rest, are what the words 'Sāṅkhya' and 'yoga' denote. So this is not an off-topic digression.