Even if I hadn't already done my own version of FR, I'd not use the 4E version since I am currently running a campaign set in Mulhorand/Unther, and I don't like what they are doing to the region.
My version of the FR doesn't use much of the 3E events and changes, about the only things I used were the invasion of Unther, the Thayan enclaves and the troubles in Amn (which were solved by my players leading a crusade to free Murann and eradicate the monsters). And in an earlier campaign I used the Sahuagin threat from a novel series. But King Azoun is still alive, Shade has not returned (and may never), Blackstaff still lives (not that he has any importance other than some cameo appearance)
My Forgotten Realms are not the high-fantasy place of 3E, they are closer to sword&sorcery, based upon a mix of backgorund and rules changes (for 3E):
- Teleport spells are only accessible to a NPC prestige class like the wayfarer. Dimension door, which is available, requires line of sight to the destination. If a party needs to travel great distances in a short time they can use portals, but those are often in remote and dangerous locations, and not always working - basically, DM controls when such portals can be used.
- Raising the dead requires divine intervention and is rare, not something that a cleric can do reliably. (Though players can expect that their PC will not get killed without a warning and a way out (even if that way out may be "surrender and get captured".) )
Also, I do not feel I have to use the levels of the NPCs as written in the books. If no one attacks Elminster he can be a 5th level wizard with good press for all I care, and Blackstaff might be a better politician than mage (which also explains why the NPCs do not solve all the problems).
Setting-wise, there are much less magic items around. There are no magic item shops, and magic items tend to be powerful, and with their own history and plot hooks, and growing in power/unlocking their true power through quests or other adventure-related processes as their wielder grows in power. No "I commission a +3 flaming shock longsword and have my plus 2 armor upgraded". So, PCs tend to have few, but powerful magic items, not a plethora of utility items. Scrolls and potions (low-level ones) are fairly common though, but usually not sold to just anyone, but hoarded for allies and members of the church who made them.
There is less treasure to be had by adventuring. Orcs, bandits and monsters haven't hordes of gold around, and an average adventurer does not make more gold than an average mercenary. Real money is made by nobles (through land and taxes) and merchants, not killing creatures and taking their stuff. In fact, I don't bother with exact gold and treasure amounts anymore - PCs can afford a certain lifestyle, with all it entails, or not. Since you can't order magic items, most of the reason to require a detailed amount of treasure is already gone anyway. Rewards like lands, titles, trade rights/monopolies are used to grant "lifestyle upgrades".
The realms are far less egalitarian than their official versions. If you are not a noble (or a filthy rich merchant in a land without a strong aristocracy) you are not worth much in lands with nobilty in charge. Serfs are slaves in all but name, and their use is widespread in the north, and in the heartlands. Slavery is common in all southern countries from Calimshan to Semphar.
There are few if any good rulers - most have to compromise their morals to rule a country.
There are few if any evil or good people - most are neutral. Good/evil people are usually religiously motivated. A bit of altruism alone does not make anyone good, nor does killing someone make anyone evil. There are no "good races" - only neutral ones. Same goes for evil races, but those are usually under the thumb of evil priests and traditions.
The humans are the dominant race. Elves and dwarves are on the decline. No thundertwin event, no "return from retreat".
Elves have a stagnant culture, rooted in age old traditions, unable to change fast enough to keep up with human innovation. Their items are still some of the most beautiful, but their magic has been surpassed by human zeal for innovation and risk taking, especially in Halrua, who added 1500 years of arcane research to the 3000 years of magical research of ther Netherese ancestors, and in Thay. Elves still have the reputation to be the realms' best and most powerful mages, but that is not true anymore (and part of the reason they hole up in Evermeet, and keep non elves out).
Dwarves are dwindling in number through low birth, and high losses in wars. They are the best metal and stone workers - elven steel may be prettier, but not better, and elven smiths never really went beyond chainmail and longswords - but again, humans are catching up.
From what I hear, 4E Realms are not to my taste. I'll buy the book, and check the changes out, but I doubt I'll use much of what they offer - I already used very little from the 3E campaign set.
One of the main reasons is that all my campaigns so far are set in the same continuity. New campaigns start with the changes the older campaigns caused, and PCs may meet common NPCs or former PCs. A time jump of 100 years would wreck that. I already ignore the novels, and stopped reading them, I already tailored the FR to my taste, so what would I gain by ditching my own countless detailed NPCs and plots and places for a "new realms"?