Updating sum and average range references based in a cell refefernce

I have sum and average formulas that show different metrics from prior years and compares them to the same metric for the YTD of the current year.  I want the prior years to display the same YTD information based on the week number of the current year.
For example, for the prior years the data is in columns B:BB.  For the current year(Week 33) the data is in columns B:AH.  Again, for example purposes, I have a formula =sum('2018'!B4:BB4) that shows me the sum of the entire year of 2018.  I need the formula to say =sum('2018'!B4:AH4) and th4 AH reference be automatically update each week based on a cell containing the week # or column reference.