How honest do you have to be I your W4?

You do not have to be honest at all. The only thing you have to do is pay your tax on time. The IRS does not care whether that's through withholding or estimated tax payments or if one spouse pays the full amount of your joint bill.

Yes, that's perfectly fine. In fact its how you are supposed to fill out a W4 for a married couple with both spouses working.

You can do it that way, no problem. Only thing you can't do is claim exempt if you are not.

Unless you significantly UNDER withhold, the IRS never even asks to see your W-4. It is ONLY a guide for your employer to determine withholding. You could tell your employer to send your ENTIRE check to the IRS and the IRS wouldn't care.

Note: There is actually a line on the W-4 to list a $ amount to withhold above what would otherwise be withheld based on the rest of the numbers on the form.

You can claim single on the W4 form--or any other status you like. The IRS doesn't care what you claim on that form--only what you either pay or overpay at the time you file your return. In fact, you can have extra withholding tax taken out of your pay at any time with the form. Or NONE. (the W4E form is the one that allows this.) 

You can have too much withheld and IRS won't complain.

i would be honest if you dont want to get in trouble for it

You can actually put anything you want to on the W-4 form.  You don't have to be "honest".

HOWEVER - if you make a mistake that causes you to OWE too much on your taxes at the end of the year, then you could get a penalty for not correctly estimating your tax liability.

Claiming single when you are actually married is not likely to cause that kind of problem.  

You should select "married, but withhold at higher single rate". That's honest, and you get what you want.

There's already a box for "married, but withhold at single rate". The W4 never goes anywhere but to your employer and the payroll processor, so you can put anything you want on it.