Step 1: Task:
We need the stated aim of the Government of India Act, 1935, with regard to caste and tribe. We will match each option to what we know about the law and keep the one that fits.
Step 2: Background:
Colonial officials wanted a clear list of groups that suffered the worst social disadvantage. The 1935 Act put such a list into law as a schedule. That is the origin of the terms Scheduled Castes and Scheduled Tribes.
Step 3: Test option 1:
Naming OBCs as a constitutional category belongs to the post-Independence period. The 1935 law has no such purpose, so we reject this option.
Step 4: Test option 2:
The Act did not exist to support academic study of the caste order. It was a governing law, so we reject this option.
Step 5: Test option 3:
A formal legal list tends to harden caste labels. It does the opposite of making them fluid, so we reject this option.
Step 6: Test option 4:
A legal recognition of the lists of castes and tribes is exactly what the Act did. We accept this option.
Final Answer:
Option 4 is the only one that matches the purpose of the 1935 Act. \[\boxed{4}\]