The most likely reason is that each employer withheld your taxes at the rate you would be liable for if you only had one income for the house.
Example. You are married and earn $45,000 at of your job with a tax bracket of 12% ($4,989) and he earns $45,000 at his job where his employer withheld $4,989 as well. So each employer withheld the correct amount based on what you earned with them. In total they withheld $9,978 .
However, when you combine your wages from all 4 jobs, your income is $90,000. This puts you in the 22% bracket for part of your income. So, your total tax liability would be $11,397 This would mean you had $1,419 too little withheld and would owe it when you filed your taxes.
You may want to complete a new W4 form for the one you make the most for or even for 2 of them. You will want to complete section 2 and probably section four to have an additional amount withheld every pay period.
You can calculate the amount extra to be withheld, if your job situation is not expected to change, by dividing your $900 (or more if you would prefer to have a refund and not risk owing again) due by the number of pay periods you have in a year (usually 26 or 52) and entering that on line 4c.
Depending on the state you live in and if they have a flat rate or a graduated rate like the federal government does, you may or may not need to make changes.
In the meantime, if you are unable to pay your taxes in full, you should still file by April 18th, but you can apply for a payment plan from the IRS. When you get to the end of TurboTax, prior to hitting file, it will give you the option to apply.
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