Collecting form data when there is not a submit button

I have 4 forms each on a separate page respectively form1.aspx,
form2.aspx, form3.aspx, form4.aspx.

On top of each of the four pages there are 4 links which link to the
aforementioned 4 pages like this:

Form1 Form2 Form3 Form4

Important: Only form4.aspx has a submit button, none of the other 3
forms has any kind of submit or 'next step' button.

So, how can I collect the data from, say, form1, when the user clicks
the link Form2 or Form3 or Form4 to continue?

I know that I can use session for this case, but the problem is, since
there is no submit button on form1, form2 and form3, what triggers the
action of data-collection from these 3 pages when the user leaves them
by clicking the links at the top of the page?

I don't think there is an onLeave function, is there?

Any idea? Thanks a lot!

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