Hello, I'm looking for a good hard, challenging route. 4 days, 3 nights with huge rewards. Big cliffs, amazing waterfalls stuff like that. Any advice?

I'll have to keep an eye on this post. Sounds like fun.

A route in the Vento unit may fit the bill. Check out the possibilities with EPs 61, 62, and 64.

Try the Q from Prairie Portage. to Agnes for Louisa Falls and high bluffs from there down agnes. to Kwenippi,l then left and back to Prairie Portage down lakes just west of Agnes. Some real portages to challenge you and long paddling days as well. See your maps for details of the travel you will do.

Might look at the north side of the Gunflint Trail.
Enter at Hungry jack (Duncan - Entry #60) down the stairs (80 rods) past the waterfall - Rose, Rove, Mountain, Drop south thru Clearwater to West Pike and then south to Pine. Go back south to Johnson Falls - then west to Canoe, Alder, East Bearskin, and Moon. Finish up in Flour and back to Hungry Jack.
Great water falls - (Duncan, Johnson) high bluffs, good side trip hiking where portages intersect the Border Route Hiking Trail.
Totals of about about 32-35 miles, over 2,371 rods and 12 hours of "on the water" time.

Or, check out the Echo Trail - Entry Point #16 - Moose River North
In Moose River North, up to LLC, east through Iron, Curtain Falls, and on past the Crooked Day Bays. Head south thru Wednesday Bay to Lower Basswood Falls then take the Horse River to Horse lake and head south thru Tin Can Mike, Sandpit and out Mudro.
This books out with totals at 47+ miles, 1,562 rods and about 18+ hours "water time".
Might be too long for your time frame but with more "water miles" you could make it.

Pine to canoe will get your heart pumping regardless of athletic level. That first path granmaL posted is very challenging. Throw royal lake into that mix. The wild rice patties are pretty neat.