The four payments do not have to be equal. So, yes, you can increase your Sep and Jan payments.
The 4 payments must be equal rule is a special situation, to avoid an underpayment penalty for "front loaded" income and is not applicable to your situation. Example: taxpayer has large income in the first quarter (e.g. stock sale). Rather than paying all the tax on the sale, in the first quarter, he can make 4 equal quarterly payments and avoid a penalty.
