Reference VAT declaration

The number is usually stated as the "Reference number of the shipment" or the "Receipt number" on the pDF receipt that you could download after submission. 

I cannot find this receipt on the Intervat website, but when I try to resubmit my declaration (manually on Intervat), the website informs me that I have already submitted a return and that I must fill in a reference number if I wish to submit a corrected return.

Go to Intervat. There you will see all your returns.
Under the column "possible actions" you will see a pDF "download the receipt confirmation". Click on it and you will receive your receipt confirmation.

I know where to look, but my last return listed there is from mid-October (Q3 return). The Q4 return is not listed.

Then take a look at MyMinFin. Under "My documents," you will normally also find your receipt.
If this is not the case, your vAT declaration may not have been submitted correctly.

Tip: download your receipt immediately after submitting your declaration.

https://financien.belgium.be/nl-BE/E-services/Intervat/hoe-intervat-gebruiken/return-verbeteren-reference#q1