I understand that one could generate lexer and parser given the antl4 grammar but Is there a way to generate builder using the antlr4 grammar? That way client can use the builder to construct the possible structure specified in the grammar while the server can use the generated parser to parse the structure.
There is, yes. Such a sentence generator can walk the ATN and create sentences according to the grammar (see my antlr4-vscode extension of how this can be implemented). However, unless you have a very simple grammar with no recursions or iterations, you will probably not be able to generate a fixed set of sentences, since there are infinitive possible combinations.