If you want to stay 4 months why do you need to do an extension for your first entry? You would be safer staying 30-40 days before exiting and returning after a short stay away then taking the extension on your second entry.
Get your Non-O before entering Thailand and that gives you your 90 day stay in Thailand with no requirement for proof of onward travel, no risk of being questioned by Immigration on entry and no need to visit Immigration for an extension. Make sure your return ticket gets you only 89 nights in Thailand as your day of arrival is day 1.
If you are not on a Non-ED Visa but on an Extension of Stay then that may create a difficulty for you as this type of EoS has to be cancelled. Don't go interfering with stamps in your passport as that can get you into even greater trouble if that is what you mean by voiding it.