There are multiple forts called 'Harihar' / similar names in Maharashtra (examples: Harihar Fort near Neral/Karjat region, Harihar Fort in Baglan region, Harishchandragad is different). Please confirm the exact fort (or share a nearby town).
Do you mean a total budget of INR 4,000 per person for the entire 5–6 day round trip (transport + stay + food + entry)? Please confirm currency and whether 4k is per person or total for a group.
On a tight budget would you accept very basic lodging (cheaper dorms/railway station overnight) and slow trains to keep costs low, or do you want a more comfortable trip even if the budget needs to rise?