Tax Allowances

That would work for a full year if you take standard deduction. If you itemize you might be able to get away with more than 4. This assumes you withhold as married.

For this year, you might want to withhold more because of the part of the year your wife is working.

If you get paid 2x a month, each withholding allowance allows for about 160 per paycheck in money that does not have federal income tax withheld on it. So if you went from 2 to 4, you would have about 320 per paycheck additional that does not have anything withheld on it. Without knowing how much you make per pay period, I can't give you more info.

There are plenty of withholding calculators on the internet that you can play around with.

This seemingly simple topic is one that is fraught with potential pitfalls. Every dollar your wife earns will be subject to tax at your married filing joint marginal tax rate. Some of the income she earns will be subjected to withholding to use the 10% and 15% tax brackets that your withholding also uses. So you could end up with 2 times the amounts of withholding made with respect to those brackets as you should.

Your wife's income could require you to increase the amount being withheld in order to avoid an underpayment. Much depends on your income levels and your marginal tax bracket.