I would go from thier last intake of food which would be the bottle, but are they hungry? Try it out and see what they eat, and go with their need. If they are not hungry 4 hours from the solids then leave it that extra hour. Does that impact on their sleep at all? Do they still get time for activity between lunch and nap if they wait? You just want to avoid them getting over tired and falling asleep in their chair, or being too tired to work on their solids properly.