Please note that TTS says it right here, so pronounce fyrtio like if it was förti.
On the "swedish in 3 minutes videos" she says it in a different way.. is it wrong? I thought those vids were awesome..
This is an exception, in other words starting with fyr-, y is pronounced just like usual.
You could say either with the "o" at the end or not. Both work.
I guess that we usually don't say the "o" at the end when speaking fast.