Talk to your priest, but in this situation I would imagine that the Catholic’s obligation to attend Mass on that particularly 4th weekend would be abrogated, i.e., the priest would ‘lift’ the obligation due to the fact that the person would not be able to find a Mass to attend outside of the person’s work obligations for that weekend.
Now if for example the person worked 8 hour shirts 8 a.m. to 8 p.m. Sat and Sun that weekend, and the local college offered a 9 p.m. Sunday Mass (as for example at UVA, Mr. Jefferson’s university during the school year), then the person could probably attend the 9 p.m. Sunday Mass after his work is over if he lived in the area.
But in most places the only available times for Sat and Sunday Masses range from say 4 to 5:30 p.m. Sat and 7:30 to 1 p.m. or so on Sunday, and there’s no way a person working the above schedule could make any of those Masses. The rule is, if something is impossible, one is not ‘held’ to it, so really the priest doesn’t even need to lift the obligation for something ‘impossible’, but it would probably make a person feel better to hear it from their priest.