That's a puzzling claim, because in Polish you have trzydzieści, czterdzieści, and only then the endings start changing, pięćdziesiąt, sześćdziesiąt... and I thought that probably was related to five and above taking the genitive plural form. So you can see why that claimed form for 40 puzzles me.
Ukrainian also takes genitive for the numbers 5, 6 etc.
I probably wasn't clear enough in what I was saying. Consider the numbers twenty and thirty. двадцять, тридцять both end the same way, in -дцять, note in particular the soft ть at the end. Now compare this with the numbers 50, 60, 70 and 80... they end in -десят, with a hard т. Have you wondered why there is a difference?
"сорок" comes from Turkish word for forty, "kırk". Forty had (and still kinda has) a spiritual importance for Turkic people. It remains as a relic from the days of Khazar Khanate