In plain words
one must then seek that state from which, once reached, there is no return, taking refuge in that very primal Puruṣa from whom this ancient manifestation has streamed forth.
Word for word
Then that state [is] to be sought, in which, having gone, [they] do not return again: "And in that very primal Person I take refuge, from whom the ancient going-forth has streamed."
ततः पदं तत् परिमार्गितव्यंयस्मिन् गता न निवर्तन्ति भूयःतमेव चाऽऽद्यं पुरुषं प्रपद्येयतः प्रवृत्तिः प्रसृता पुराणी
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam· bhāṣya
ततः पश्चात् यत् पदं वैष्णवं तत् परिमार्गितव्यम् , परिमार्गणम् अन्वेषणं ज्ञातव्यमित्यर्थः । यस्मिन् पदे गताः प्रविष्टाः न निवर्तन्ति न आवर्तन्ते भूयः पुनः संसाराय । कथं परिमार्गितव्यमिति आह — तमेव च यः पदशब्देन उक्तः आद्यम् आदौ भवम् आद्यं पुरुषं प्रपद्ये इत्येवं परिमार्गितव्यं तच्छरणतया इत्यर्थः । कः असौ पुरुषः इति, उच्यते — यतः यस्मात् पुरुषात् संसारमायावृक्षप्रवृत्तिः प्रसृता निःसृता, ऐन्द्रजालिकादिव माया, पुराणी चिरन्तनी ॥ ४ ॥
In plain words
'Thereafter' simply means: after that. That state which belongs to Viṣṇu is the one to be sought — 'to seek' here just means to search for, and it also means to come to know it. It is a state which, once you have gone there, once you have entered it, you never come back from, never again to saṃsāra. So how does one actually seek it? The verse itself answers: 'Him alone,' the very one that word 'state' was pointing to — 'the original one, existing from the very beginning, the original Person, I take refuge in.' That is how it is sought: by taking refuge in him, precisely. And who is that Person? Here is the answer: it is the one 'from whom', from which Person, the whole activity of this illusory tree of saṃsāra has spread out and poured forth, like the trick of a magician, an ancient, age-old māyā.
Close to the Sanskrit
'Thereafter', afterward, that state which is Viṣṇu's is to be sought out — 'parimārgaṇa' means search, seeking; 'is to be known': this is the meaning. In which state, having gone, having entered, they do not return, do not come back again, to saṃsāra. How is it to be sought? — [the verse] says: 'him alone, and,' [namely] the one denoted by the word 'pada' — 'the original', [i.e.] existing at the beginning, 'the original Person, I take refuge in' — thus, it is to be sought by way of taking refuge in him: this is the meaning. Who is that Person? — it is explained — 'from whom', from which Person, the emergence of the illusory tree of saṃsāra has spread, has issued forth, [that] māyā, ancient, age-old.