How about putting part of your query into a sub-select (or maybe even a separate query). Like this:
SELECT
orig.employee_id,
IIF(orig.wk_ending_dt=PreAgg.max_wk_ending_dt),"Last Wk",
IIF(orig.wk_ending_dt=PreAgg.max_wk_ending_dt)-7,"2 Wks Ago",
IIF(orig.wk_ending_dt=PreAgg.max_wk_ending_dt)-14,"3 Wks Ago",
IIF(orig.wk_ending_dt=PreAgg.max_wk_ending_dt)-21,"4 Wks Ago")))),
orig.hrs_worked,
orig.move_count,
orig.moves_per_hour
FROM (
SELECT
d2s_loader_performance_tbl.employee_id,
max(d2s_loader_performance_tbl.wk_ending_dt) AS Max_wk_ending_dt
FROM d2s_loader_performance_tbl
GROUP BY
d2s_loader_performance_tbl.employee_id
) PreAgg JOIN d2s_loader_performance_tbl AS orig ON
PreAgg.employee_id=orig.employee_id
WHERE orig.wk_ending_dt In (
SELECT DISTINCT TOP 4 wk_ending_dt
FROM d2s_loader_performance_tbl
ORDER BY wk_ending_dt DESC)