I'm used to achieve this under Plone 3 by override base_edit.cpt file under portal_skin and make columns empty:
<metal:head fill-slot="column_one_slot"></metal:head>
<metal:head fill-slot="column_two_slot"></metal:head>
I have tried to achieve this with Plone4 and the result is quite weird: I have two empty columns and the edit mode doesn't stretch (stay in the middle of the page).
In Plone 4 you can disable the columns by setting a couple variables on the request:
<metal:override fill-slot="top_slot"
tal:define="disable_column_one python:request.set('disable_plone.leftcolumn',1);
disable_column_two python:request.set('disable_plone.rightcolumn',1);"/>
I think that's because plone 3 used tables as core page structure while plone 4 uses sunburst that uses Deco GS.
Probably the fastest way (not THE way :P) is to force the width of the columns trough CSS (you can push CSS using style_slot into base_edit.cpt).
Anyway, I see in
./plonetheme.sunburst-1.0.3-py2.6.egg/plonetheme/sunburst/skins/sunburst_templates/main_template.pt
that columns class/width should be calculated by getColumnsClass
of sunburst_view
. Probably is a bit buggy (I didn't debug it though).
HTH