The redeem 4 nights, get the 5th night free is still in effect. But it doesn't work under a cash and points reservation. Do you have 120K points in your MR account? If you are trying to use 100K points then are most likely SOL. Some MR agents will make the date change to your reservation without changing the amount of points required under the old category, but SOP is that a change in dates make it a new reservation thus requiring higher points.
If you don't have the 120K points required for 5 nights @ a Cat 6 property, you can buy 20,000 MR points for $250 from Marriott's website. That would enable you to redeem the 4 nights and get the 5th night free, provided the hotel has 5 nights available on your dates.
https://buy.points.com/PointsPartner...EN&product=BUY
Hope this helps.
Regards,
RIP...