Just out of curiosity, how much NJ tax is this client paying this year?  The Corruption Capital of the Country still requires ES payments if the amount is as low as $400.  I think they got that from IRS, before the federal requirement was raised a couple decades ago.  It probably costs them more to process those $100 payments, than it does for them to borrow short-term against April collections.  

And is the client making the payments from a checking account that pays no interest and has an average balance of $20,000?  There is no law against making all four payments in April of the current year.  Even if the ES tax is $1,000 and he is earning 5% on a money-market account, he would forego only $25 or so interest by paying all at once.  That might be $15, after taxes.  And you're going to hold his hand all year for free, just so he feels better about it?