PDF 1.4 - order of form fields (tab order, signing order)

I have a question related the order of form fields:

How can I influence the order of fields?

I use signature fields and want to explicitly set which signature is signed first, second etc.

Also the tab order would be interesting. (For that since PDF 1.5 there is a 'Tabs' key however how is that done in PDF 1.4?)

Any help appreciated,

Regards,

ToM