W-4? I think you meant to type W-2.
Yes, you will pay taxes on that SE income, and since it was more than $400 you will also pay the additional 15.3% self-employment tax too. The SE tax is basically the employer side of your social security and Medicare. I would suggest you send the IRS about 20% of your SE earnings "right now" so you don't have to waste your time dealing with applying for exceptions to late fees, underpayment penalties and all that stuff. So with $12K of SE income if you send the IRS $2,400 *right now* then come tax filing time you should be fine. You can pay online at www.irs.gov/payments.
Also, since "me and my wife" did some SE work, you will each be completing your own physically separate SCH C to be included with your 1040 joint tax return. PAY ATTENTION to detail. If you will NOT be doing this same type of business in 2019, then not only will you each indicate you opened your business in 2018, you will also indicate that you closed it in the same tax year. Otherwise, (and it depends on the type of business) the IRS will be expecting a SCH C from you the next tax year and will notice it, if you don't send them one.
