How can i claim share of ancestral property?

W4 has right over the share of her husband and hence she can file a case by consulting an experienced advocate in the field.

Should she file a declaratory suit or partition suit? He already had 1/4 property in her name, she only doesnt know which part of property belongs to her.

is it legally allowed for W4 to make a WILL for property she will inherit a property in future? If yes will that WILL be still valid after her death?

Yes, she can file a partition suit. Even in the absence of oral partition, the property is jointly vested with her. She can sell, gift or other wise transfer her share in the jointly held property. She can make a will as well; it would be better if the Will is got registered, though  not mandatory.

But, let her at first issue notice to all co-sharers and demand for providing separate possession of the property through metes and bounds.  Generally, if a widow claims her right, due to social stigma, the brothers yield and give her share without much fuss as family honor is involved.

Just file the partition suit ; and W4 has right in the property .

Since the property has already been partitioned by meets and bounds between (S1, S2, S3 & S4) by their father wherein all of share holders are in their respective possession for the last 12 years, it is a valid partition and there is no requirement to file a separate suit for partition.