No registered will by my father. mother will be natural successor?

If your father died intestate  then his property rights devolve to his all legal heirs equally I.e wife and children.

Thanks.If father's will is in plain paper,but not registered then does that have any value?

Yes . All sucesseser will get equal rights.

An unregistered Will is also legally valid provided it was prepared following the procedures of law, i.e., the testator signing it on all pages, two witnesses attesting the signature of the attestor.

If the Will is not valid then the property shall devolve equally on all the legal heirs/successors in interest of the deceased considering the deceased died intestate opening intestate sucession.

Thanks Mr. Kalaiselvan. Can the winess be legal heirs(son/daughter)?

There is no legal infirmity for the legal heirs to be attesting witnesses to the Will.

These are two different situations created and posted by you.

Initially your query regarding share of your mother and siblings so it is amply clear that since your father is stated to have died intestate, all four of you (1/4th share of your mother) have equal share in the property left behind by deceased.

Subsequently you have changed the scenairio wherein the deceased has executed a will on an unregistered Will on plain paper (prima facie it is hypothetical and concocted story) the beneficiary of the will in self-acquired property of deceased shall have the same (property).

In the absence of a registered will by your father, the succession laws may vary depending on the jurisdiction. Generally, without a will, the distribution of assets will follow the rules of intestate succession. In many jurisdictions, the surviving spouse (your mother) may be entitled to a portion of the estate, while the remaining assets may be divided equally among the siblings. It is advisable to consult with a legal professional to understand the specific laws applicable in your jurisdiction.

Registration of a Will is optional, which do not have any bearing on the testament.

Thanks for all the responses. I reall appreciate . However, with due respects to all the valued opinions, I fail to understand why opinions vary from advocate to advocate. I discussed with local legal experts also and their opinions are also not exactly similar. How can I decide whom I should approach for formal legal service? Regards

While asking for opinion on platforms like this, you should make sure what is the problem you face and seek solutions for the actual problem.

If you post different scenario in the subsequent posts, then it is natural that you will different opinions only.