Hello,
Welcome to the ehealthforum and I am really glad to help you out. This patch which has been present for 4 weeks and is not changing in size and shape can be due to eczema, psoriasis or lichen planus. If you are sexually active then STD's also should be ruled out.
Lichen planus is an inflammatory disease that presents itself in the form of papules(flat spots) or rashes. To confirm the diagnosis I suggest you to get an evaluation done from a dermatologist.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care.