@real4uk , sorry for the confusion :
(a) your filling out of the form W-4 is only good for your employer to withhold taxes on behalf of the Federal and the State. They do this by computing taxes due on the income earned till date at that employer and is based on "estimated deductions" that you probably will claim. Many people fill out W-4 as single ( when they actually married )-- this way more of their earnings are held back as "prepaid" or PAYE taxes. The advanatge of this is that when they finally file their returns they get a larger amount back -- it is their money that was loaned to the govt. at zero interest. Some use this as a forced savings or other reasons.
(b) when you file your return , it is the real taxes computed, adjusted against the withholding and excess returned to the taxpayer. It is not a PAYE system in that respect and unlike the UK , you have to file each year unless your income is below the filing requirement ( in the region of 10-12 K or thereabouts, depending on your filing status and number of dependents etc. )
(c) if you enter the USA on December 25th, you may not have any earnings in the USA for the tax year 2021 and thus you will not need to file a return on or before April 2022. In such a case you may be better served by staying out of the USA till the 1st. Jan2022 -- taxes would be simpler ( in the USA ). UK also may not tax the first quarter of 2022 ( have to look up the USA-UK tax treaty to be sure -- some countries do claim this period though ).
Hope this clarifies the situation ( difference between the UK and the US system ).
