At 4000 points, the offer is still tightly centered on Alaska. The best sailing is Ovation of the Seas on a 7 Night Northbound Alaska & Hubbard Glacier Oceanview, with Cruise fare for 2 guests and $300 FreePlay. The tier is all two-guest inventory, and the top results are concentrated enough that the value case is easy to read.
At 6500 points, the free play steps up to $500, but the sailing mix broadens. Alaska remains the best region, with Serenade of the Seas and Ovation of the Seas leading the list, yet the tier also starts to show more Caribbean and Asia volume. That makes it less concentrated than 4000, but more flexible if you are not chasing one specific Alaska departure.
The main difference is not just the free play increase. It is the cabin and itinerary mix. The 4000 tier is almost entirely Cruise fare for 2 guests, and the strongest options are a small set of Alaska sailings. The 6500 tier keeps the same basic structure, but the broader mix gives you more room to choose between Alaska, Caribbean, and some longer-haul options.
If you want the cleaner value read, 4000 is the better breakpoint. If you want more choice and can absorb the higher point cost, 6500 is the more flexible tier.