either you're willing to run 4 searches, one for each week, which can be done like this:
| makeresults | eval earliest=<week2_begins>, latest=<week1_ends>, weeknumber=1
| append [ | makeresults | eval earliest=<week2_begins>, latest=<week2_ends>, weeknumber=2 ]
| append [ | makeresults | eval earliest=<week3_begins>, latest=<week3_ends>,weeknumber=3 ]
| append [ | makeresults | eval earliest=<week4_begins>, latest=<week4_ends>, weeknumber=4 ]
| map search="search index=en_amp_api earliest=$earliest$ latest=$latest$ | eval description=$weeknumber$"
Or you specify earliest and latest that includes all the 4 weeks
index=<yourindex> earliest=<week1_begins> latest=<week4_ends> | stats sum(eval(if(last_seen<strftime(relative_time(_time,"-4w"),"%Y-%m-%dT%H:%M:%SZ"),1,0))) as "week1", sum(eval(if(strftime(relative_time(_time,"-4w"),"%Y-%m-%dT%H:%M:%SZ")<last_seen AND last_seen<strftime(relative_time(_time,"-3w"),"%Y-%m-%dT%H:%M:%SZ"),1,0))) as "week2"
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