Thanks a lot for your reply and guidance sir . I will login to my ircc account and will change the martial status from married to common law . I will also write a letter of explanation that why i am changing my martial status as you guided me . Both you and kaibigan have been extremely kind and helpful.My comment would indeed be that if living together four years, applying as common law would be the safer route. IRCC does not - to my knowledge - deny applications for couples that (arguably or disputedly) are married and apply as common law; I'm more certain in saying that IRCC does deny those who apply as married and do not appear to have a valid marriage.
Further, as @Kaibigan noted above, the current marriage is almost certainly (in my opinion) invalid and won't be recognized under Canadian law. BUT: I've never heard of someone being 'punished' for being in a marriage that technically is bigamous but was carried out in good faith while abroad, such marriages are simply not recognized. That doesn't mean the common law part won't be recognized.
So: my suggestion would be to apply as common law, provide all info about the previous marriage, and explicitly note that it was considered legal and valid in India, but you are applying as common law. You can even say you are applying as common law BECAUSE you heard that it might not be recognized as a valid marraige in Canada - there's no downside I can see to being open about that.
If you're still uncertain: sure, see a lawyer who knows about Canadian immigration law. But I'd repeat that I don't really see any downside to applying as common law - to the extent I'd even be a bit skeptical about a lawyer who actively counselled against that.
Please let me know if any of you can guide more about this situation.
Thank you both of you .