data validadion doesn't work when worksheet is populated from userfrom??????

Hi All
I have this formula =COUNTA($A:$A)<=4 that limits amount of cells that can be populated in column A, I use data validation with "Allow costume" option and using that formula.
It works fine from worksheet it displays the message when the limit is reached but it doesn't work when data is inputed/populated from userfrom, it allows userform to put more entries than set limit 4 in this case.
Is this common issue? has anyone ever come accross something like this?

Thank You.
Dan