Dear Paul Mills2
Good day! Thank you for posting in Microsoft Community. We are happy to help you.
Generally, the number of hours a group member or resource spends on a task. You can enter whatever amount of effort you like in the equation "Completed + Remaining = Total." After that, if you change duration or assignment, effort will adjust to fit. If a day is added to duration, effort is increased by one day's worth of hours. The detailed information you can refer to Effort and duration in Project for the web (microsoft.com)
Meanwhile, we will do a test on our end in the Project web to check if the distribution of hours between each resource is 4hrs or we'll get the same result as yours. This test maybe needs a little time to finish, please kindly keep patient and we'll post back as soon as possible once the test was finished.
Besides, we also welcome other members or MVPs to share their ideas about this query in here.
Your understanding and patience will be highly appreciated. I hope that you are keeping safe and well!
Sincerely,
Stacey | Microsoft Community Moderator