Income Tax Return for salary + retainership

Treatment of the fees received depends on nature of engagement of you with the company & the kind of services rendered. If the retainership fees paid is in nature of remuneration for services rendered for business activities & if you are in full time, the fees amounts to remuneration which can be received either as salary or fees for services. Since you have received it as fees, it should be treated under head 'Profit & gains of business or profession' & taxed accordingly.

Accordingly, you can claim expenses.

hence, Form IV should be used.

Hope your query is answered.