I think Belgium law is in your favour any way, you may even get a full 4 years salary in payout, or more. Your signature on that piece of paper is going to be worth a lot to them, more than you would believe.
If your scenario is anywhere near correct ,I think most employees are fresh out of luck.
Up until last year it indeed used to be so that an employee with a substantial
amount of years of service with a Belgium based contract can get a lot of months of pay when the contract is prematurely terminated, even better with mass layoff scenario's.
But all that has changed dramatically last year, now I believe it is severely capped, the only way they would get such high payout is when they come to an agreement with UPS .