Retrieving value of column X days later

Hi,

Let's say I've got a table :

create table test (
when date,
foo numeric,
bar numeric,
alice numeric,
bob numeric);

insert into test values ('2016-01-01',1,2,3,4);
insert into test values ('2016-01-02',5,6,7,8);
insert into test values ('2016-01-03',9,10,11,12);
insert into test values ('2016-01-04',13,14,15,16);
insert into test values ('2016-01-05',17,18,19,20);


What I would like to do is the following :


(1) Given "select alice,bob from test where foo=1 and bar=2"  I would
like to return the values of alice, bob  *and* the value of foo four
days later (i.e. "17" in the above example).


(2) But there may be times where there are insufficient data points,
and so I would want to retrieve the last available value (e.g. "select
alice,bob from test where foo=9 and bar=10", there is 4 days hence,
therefore it would return the last available, i.e. "17" in this
example, even though that is only 2 days hence).


Any ideas welcome !

Thanks !