Is it an ethical practice for a 501c3 to approach a potential landlord with the idea that if they lease to them a building for $4000 a month, but only charge the non-profit $1000 a month, the landlord gets a $3000 write off that was never collected?

This was an actual statement from the President of a 501c3.

Yep - no problem, as long as the $4K rent figure is a good estimate of the true fair market value of a month's rent. The landlord is indeed making a gift, as he could rent the space for $3K more per month to another party. The 501c3 should provide a receipt for the gift of the $3K/month rent subsidy.