In Gurbani there is a lot of mention of Chaar Padarath (4 Cardinal Substances). These Chaar Padaarath are such that if a person possesses all of them, he can be considered to have attained every worldly happiness. So what are these four Cardinal Substances:
ਮੇਰੇ ਮਨ ਜਪਿ ਹਰਿ ਗੁਨ ਅਕਥ ਸੁਨਥਈ ॥
ਧਰਮੁ ਅਰਥੁ ਸਭੁ ਕਾਮੁ ਮੋਖੁ ਹੈ ਜਨ ਪੀਛੈ ਲਗਿ ਫਿਰਥਈ ॥੧॥ 1320
(O my Mind, chant Vaheguru and listen to Gurbani (Akath Gunn). Dharma, Arth, Kaam and Mokh all, follow behind the servant of Vaheguru)
ਸਾਧਸੰਗਿ ਆਰਾਧਨਾ ਹਰਿ ਨਿਧਿ ਆਪਾਰ ॥
ਧਰਮ ਅਰਥ ਅਰੁ ਕਾਮ ਮੋਖ ਦੇਤੇ ਨਹੀ ਬਾਰ ॥੨॥ 816
(Worship Vaheguru the treasure of all qualities in Sadhsangat (holy congregation); it does not take a moment by Vaheguru to bless with Dharma, Artha, Kaam and Mokh.
In light of the above pankitis, the four padaraths according to Gurbani are:
Dharam - Fulfilment of duties including karmic debts.
Arth - Wealth
Kaam - Tasks, works.
Mokh - Salvation
It is rare for a person to obtain all four cardinal substances during a lifetime. If one has Arth (money), one may lack Mokh (salvation). If one's wishes are granted, then one may not be able to fulfill one's duties towards the society, family and religion. Gurmukhs who japp Naam are able to attain all 4 basic and cardinal substances.
How does one get char padarth?
ਚਾਰਿ ਪਦਾਰਥ ਹਰਿ ਕੀ ਸੇਵਾ ॥ ਪਾਰਜਾਤੁ ਜਪਿ ਅਲਖ ਅਭੇਵਾ ॥ (108)
(The four blessings are obtained by doing service of Vaheguru. Paarjaat the gift-granting tree is obtained by chanting the unreachable Vaheguru)
ਚਾਰਿ ਪਦਾਰਥ ਜੇ ਕੋ ਮਾਗੈ ॥ ਸਾਧ ਜਨਾ ਕੀ ਸੇਵਾ ਲਾਗੈ ॥ 266
(If someone is seeking the four Padarath i.e. substances, he should engage in the service of Sants)
ਰਾਮ ਨਾਮੁ ਰਸੁ ਰਾਮ ਰਸਾਇਣੁ ਹਰਿ ਸੇਵਹੁ ਸੰਤ ਜਨਹੁ ॥
ਚਾਰਿ ਪਦਾਰਥ ਚਾਰੇ ਪਾਏ ਗੁਰਮਤਿ ਨਾਨਕ ਹਰਿ ਭਜਹੁ ॥੪॥੪॥ (799)
(The taste of Raam Naam is the house of all tastes, therefore, serve the Lord to obtain this divine taste. There are four main substances (Padarath) and you would obtain all of them, if you chant Vaheguru)
Does doing seva on saints means serving an actual gurmukh, serving guru jee by doing naam simran or by doing seva in the Gurdwara Sahib?.
What does it meaning by serving the Sants?
The most important thing to do is Naam Japna. In Siri Sukhmani Sahib Guru Sahib states that the real sewa of Sant is to Japp Naam:
ਸੰਤ ਕੀ ਸੇਵਾ ਨਾਮੁ ਧਿਆਈਐ ॥
(Contemplating on Naam is what constitutes serving the Sant)
In the light of this pankiti, the "ਸਾਧ ਜਨਾ ਕੀ ਸੇਵਾ" should be mean to japp Naam. But this does not mean that one should not do sewa at Gurdwara Sahib or serve Gursikhs. We must strive to do this but the meaning of "ਸਾਧ ਜਨਾ ਕੀ ਸੇਵਾ ਲਾਗੈ ॥ " seems to be Naam Japna.
As per the above Gurmat Vichar, chanting the true Naam is the source of all happiness in this world and the next one. The four cardinal substances in this world too are received by chanting Naam.