I don't think you can fold a correllated sub-query used to obtain the
result of an aggregation function for a filter in a where clause.
So the 'select max(x4.effdt)...' has to stay.  Folding the 'select
x3.setid ...' is doable, however:

select x0.setid ,x0.cust_id ,x0.name1 ,x0.roleuser ,

sum(x1.bal_amt) ,x2.cr_limit ,x2.cr_limit_rev_dt ,x2.cr_limit_range ,
         x2.credit_class    ,x0.currency_cd
from  "sysadm".ps_customer x0,
       "sysadm".ps_cust_data x1,
       "sysadm".ps_cust_credit x2,
       "sysadm".ps_set_cntrl_rec x3
WHERE x0.cust_status  = 'A'
     AND x0.bill_to_flg = 'Y'
     AND x0.cust_level != 'P'
     AND x0.setid = x3.setid
     AND x3.setcntrlvalue = x1.business_unit
     AND x3.recname = 'CUSTOMER'
     AND x0.cust_id = x1.cust_id
     AND x2.setid = x0.setid
     AND x1.cust_id = x2.cust_id
     AND x2.effdt = (
                             select max(x4.effdt )
                             from "sysadm".ps_cust_credit x4
                             where x4.setid = x2.setid
                               AND x4.cust_id = x2.cust_id
                               AND x4.effdt <= TODAY
                               AND x4.eff_status = 'A'
                           )
group by x0.setid ,x0.cust_id ,x0.name1 ,x0.roleuser
    ,x2.cr_limit ,x2.cr_limit_rev_dt ,x2.cr_limit_range ,x2.credit_class
    ,x0.currency_cd ;

You can also get rid of all those levels of nested parenthesis since
it's all AND linkage logic.
It's much easier to follow the logic that way.  ;-)

Art S. Kagel