Bg 4.15

evaṁ jñātvā kṛtaṁ karma
pūrvair api mumukṣubhiḥ
kuru karmaiva tasmāt tvaṁ
pūrvaiḥ pūrva-taraṁ kṛtam

Synonyms: 

evam — thus; jñātvā — knowing well; kṛtam — was performed; karma — work; pūrvaiḥ — by past authorities; api — indeed; mumukṣubhiḥ — who attained liberation; kuru — just perform; karma — prescribed duty; eva — certainly; tasmāt — therefore; tvam — you; pūrvaiḥ — by the predecessors; pūrva–taram — in ancient times; kṛtam — as performed.

Translation: 

All the liberated souls in ancient times acted with this understanding of My transcendental nature. Therefore you should perform your duty, following in their footsteps.

 

BG 4.15

Krishna is saying that it is time tested process, like a doctor says that so many people has been benefited so you take it up you will also be benefited. Krishna Consciousness is neither old process nor new process it is eternal process and also it is going in the history for long – long process. Krishna is talking about karma yoga which is culminate in Bhakti yoga.

Srila Prabhupad in purport: Foolish persons or neophytes in Kṛṣṇa consciousness often want to retire from activities without having knowledge of Kṛṣṇa consciousness. Arjuna‘s desire to retire from activities on the battlefield was not approved by the Lord. One only needs to know how to act.

Krishna is saying there is a way to act by which one can be free from bondage, it is not that just by doing action one will be bonded and just by inaction someone will be free from bondage. It is not work that is binding it is motivation that is binding. Example given in Srimad Bhagavatam, if a person takes too much of milk he will get digestive disorder, but the same milk if someone will take in the form of a curd he will be cured from digestive disorder. Same milk which is transformed into curd can cure the disease. 

Similarly work for fruitive result will cause bondage, but work with detachment done in devotional service will make one free from bondage. So, simply if one knows how to work, one is free from bondage. Karma Yoga culminate into Bhakti Yoga.