None of them are "religions". Religion is an Abrahamic concept. India doesn't have it. The word Dharma has totally different meaning.
Read Rajiv Malhotra's book "Being Different" to learn more(i bought ebook version for mere Rs.250, text version cost too much 450 or so). These 4 so-called "religions" were created by Britishers. Even Buddhism never was a "religion". Ashoka tried it hard but failed miserably. India works on सापेक्ष धर्म, Saapeksh-Dharma(secularism), which means "In accordance with Dharma". These words are present even in English version of Indian Constitution. (the word "Dharma" is secular because its universal. It is not based on "One God" or any particular sampradaya. It doesn't even require a God, even an atheist Yogi is totally Dharm-saapeksh.)
This word "Secularism" was added in 1975, into indian Constitution by Indira Gandhi, under Emergency rule. During emergency, parliament was dismissed and whole power remained in two hands - PM(Indira) and President(Fakhruddin). Before inserting such Constitutional amendment, you need parliement debate and voting. She took neither of it & ordered Fakhruddin to sign an unilateral ordinance(a single person VETO).
This is why none of her opponent parties has accepted this claim of "secular india", which remains a farce. For those who are wondering what indian Constitution was originally in 1947, they should see this pic(photos of Ram,Sita & Laxman on 1st edition of indian Constitution.)
In Samvidhan Sabha debate, B.R.Ambedkar said - "Secularism has no relevance for India. It was invented for European religions. I refuse to insert this word in Constitution as its fit in our indian context & philosophies".
(read Supreme Court's decision on "Constitution" in 1994. It said - "Hindutva(same as Hinduness/core-principles) is nectar of indian Constitution").
Sikhism's core books are copied from 3rd Adhyaay(अध्याय) of Markandeya-Upnishad. Gurudwaras in Punjab were all run by Brahmins till 1924. In 1925, a law was passed by some people in Sikh-Prabandhak Committee declaring that "From now on, Sikhs are not hindus. All brahmins should be removed from Gurudwaras. They are not welcome anymore in Gurudwaras". This is where seeds of Khalistan Terrorist movement were sown. (In other words, Gurudwaras were merely temples till 1924. They were declared non-temples in 1925)
Rajiv ji's other book "Indra's Net" explains how Buddhism traveled from India to China. And he also gives proof of how West copied concepts of "Hologram", "Virtual reality" from Zen & Chinese Buddhism. He traced whole chain of knowledge movement in East and West. (i have purchased that book too, but his books are so huge and full of detailed events, it takes long to read them, i will finish BD soon, then begin IN. I will keep posting here.)
RM has also explained a big difference in Western and Indian way of "thinking". Abrahamic institutions are obsessed with controlling every aspect of citizen's life by text-laws. This is due to their inherent abrahamic belief that "World is made from fragmented parts. And they were artificially glued together to create the universe". This belief causes them to think if they don't control life of individuals(parents, kids, relations, marriages, divorce), the society will fall apart and whole nation will disintegrate. This belief is not there in Dharmic Civilizations. In indian belief system - "World is made of just one particle(there is much more to it, but due to space,cutting ) ,which fragmented into infinite particles, entities. Even if universe appears fragmented, in real it is not so. They are in constant connect with each other". This belief gave stability to Eastern civilizations, while Abrahamic Civilizations became Control-Freaks! (see the debate on "whether Government should control birth of kid, abortion" in West). That's the reason Anglo-indians hate Modi. Their education has been one-dimensional, while Modi knows these both aspects. Modi is a dhoti-wearing, gavathi(गावठी) nightmare, embarrassment(शर्म) for them. Similar to how they used to treat Vajpayee.