To the people with "lost coins":
I would send all coins (when available again) to Polo, delete everything and start with a fresh wallet and new sync.
Then just send coins back and finally you should be fine. 
If your backup is safe, you're safe! Those NAVs don't get lost or something...

I would not recommend that since it is not needed at all.If you "lost coins" it is just a display error of the wallet. Make sure you are on 4.0.1.1
Here is how to update:
We just released NavCoin v4.0.1.This new version fixes a
critical block validation issue which led to network incongruences between nodes on the last days.
EVERY 4.0 user should update to this version immediately.
Downloads available at:
http://navcoin.org/downloads/INSTRUCTIONS TO UPDATE
1. Backup your wallet.
2. Close NavCoin Core if it is open.
3. Remove the folders 'chainstate' and 'blocks' from:
WINDOWS -> C:/Users//AppData/Roaming/NavCoin4/
MAC -> ~/Library/Application Support/NavCoin2
LINUX -> ~/.NavCoin2/
4. Download new version, install and run.
Always make sure to have a backup of your wallet.dat!
If you have the display error and your balance shows wrong, you can always check your balance in the block explorer.
https://chainz.cryptoid.info/nav/tldr: No coins are lost, only a display error. Update your wallet and always keep a backup of your wallet.dat
But if i would be one of those users, specially who come from NAV 3.x or older, i would do it that way.
Placebo effect maybe... but start on a fresh wallet can't be wrong imho.