Do Sadhu, Sant, Rishi and Muni mean the same thing? How are these terms defined?
मनुमेकाग्रमासीनमभिगम्य महर्षयः ।
प्रतिपूज्य यथान्यायमिदं वचनमब्रुवन् ॥ १ ॥
manumekāgramāsīnamabhigamya maharṣayaḥ |
pratipūjya yathānyāyamidaṃ vacanamabruvan || 1 ||
The Great Sages, having approached Manu, paid their respect to him in due form, and finding him seated with mind calm and collected, addressed him these words—(1).
The great sages. — The word ‘ṛṣi’ means the Veda; and the word ‘ṛṣi’ is applied also to a person, by virtue of his possessing excellent knowledge of the Veda and all that is prescribed therein and acting up to these.
The Ṛṣis, sages, who approached Manu, were great; the said persons become ‘great,’ when the above-mentioned qualities become developed in them to a very high degree; just as Yudhiṣṭhira is called the ‘greatest of the Kurus’ (because he possessed, in a very high degree, the qualities that distinguished the members of the Kuru-race).— Or the sages may be regarded as ‘great,’ by virtue of their superior austerities, or of the great respect and fame enjoyed by them.
They are the authors or seers of the Vedic hymns; ऋषयो मन्त्रद्रष्टारो वसिष्ठादयः; or,
according to Yāska, यस्य वाक्यं स ऋषिः, i.e., they are the persons to whom the Vedic hymns were revealed.
अग्नीनात्मनि वैतानान् समारोप्य यथाविधि ।
अनग्निरनिकेतः स्यान् मुनिर्मूलफलाशनः ॥ २५ ॥
agnīnātmani vaitānān samāropya yathāvidhi |
anagniraniketaḥ syān munirmūlaphalāśanaḥ || 25 ||
Haying reposited, according to rule, the Śrauta Fires within himself, he shall be a silent hermit, without fires and without a house, living upon roots and fruits. —(25).
‘He shall be a silent hermit’. — The construction is ‘muniḥ syāt’, ‘he shall be a muni’; which means that he shall keep his speech under control; the man who has his speech under control is called ‘a keeper of the vow of silence’.
Again commenting on Manu 6.41 he says:
अगारादभिनिष्क्रान्तः पवित्रोपचितो मुनिः ।
समुपोढेषु कामेषु निरपेक्षः परिव्रजेत् ॥ ४१ ॥
agārādabhiniṣkrāntaḥ pavitropacito muniḥ |
samupoḍheṣu kāmeṣu nirapekṣaḥ parivrajet || 41 ||
Having departed from his house, fully equipped with the sacred things, he shall go forth, silent and wholly indifferent towards pleasures that may be presented to him.—(41.)
‘Muni’, — ‘silently,’ — speaking little.
(English translation by Ganganath Jha)
Sadhu means "good" in sanskrit. This person must be good according to scriptures, i.e. something that God likes. It is a very broad range.
Sant - I cannot immediately recognize, what is meant by that. "sata" would be an independent person, transcendental to material inebrieties (e.g. Bhagavata Purana 1.2.17). "sat"=independent, permanent. "Shanta" would mean "peaceful".
Rishi would mean "glorious", "glorified" or "glorifier", somebody who sings or composes the vedic prayers. "rich" in Sanskrit (two sounds, vowel "ri" + consonant "ch", both absent in the English) means "praise". Chaitanya Charitamrita 2.17.186 says "unless one's opinion is different in some way, he is not considered a Rishi (sage)":
Nasav rishir yasya matam na bhinnam.
Muni = philosopher, thinker. Probably a silent, introvert person. This is related to "mauna"="silence".
All are away from materialism and are progressing on spirituality however little difference is there between them
Sadhu - The who has renunciated the world and living alone
Rishi - Sadhu who has progressed a lot...and stays in his Ashram while teaching others about God
Muni - A Rsihi who lives alone..and normally does not impart knowedge..but busy in his own quest further
Sant - One who knows the truth... he may be living with family but will be busy in propagating the truth
Source : source
In the Bhagavad Gita, in response to his devotee's request as to how a man of enlightenment behaves, the Blessed Lord ( Krishna ) replied - verses 55 and 56 of chapter 2.
O Partha ! When a man completely relinquishes all desires of the mind, and is entirely contented in the Self, by the Self, he is then considered to be one settled in wisdom. He whose consciousness is not shaken by anxiety under afflictions nor by attachment to happiness under favourable circumstances; he who is free from worldly loves, fears and angers - he is called a muni of steady discrimination.
Narada was a muni. He went in/out of Vaikuntha as he pleased i.e. the highest spirituality achievable
Budhakaushik was a rishi. He wrote Rama Raksha stotras. Valmiki was also a rishi. He wrote Ramayan
Gyaneshwar was a sant. So was Tukaram. They took Rishi's work to the masses
Sadhoo is anybody who renounces worldly comforts in search of truth.
Sadhu : Who gives if asked for or not even ask but shows his/her desire i.e. one who looking god in every one came before him. Sant : Who is nothing to give. But glory he has because of his wisdom.
Rushi : One who teaches after glory. Ways & Means to survive.
Muni : Who dictate his philosophy for reason to one who approaches.
This words are from the date unknown but must be from Sanskrit when people learnt to understand widely humanity what it is _ after wildness. Definitely Sanskrit came when people understand the difference between man and animal: Pashu and Pakshi. This Sadhu-Sant, Rushi-Muni always been called or heard of like Mata-Pita, Beta-Beti, Pati-Patni or Accha-Bura. Then there ... I think _ must not be much difference in their meaning but to narrate much more wisdom in one particular act to perform,recognized or for its sanctioned attitude. This four people may or must be from one Ashram...
Muni is to be a person who is having power of मनपर्याय ज्ञान or mind reading power
Riddhi is riches , hence a Rishi is having power of rich
Siddha ...Siddhi is possession of supernatural powers
Sant word is derivative of Saint means One who is sanctified or made holy; a person who is separated unto God’s service.