index not being used

I'm running an older PostgreSQL 9.1 database. I know it's old... an
upgrade is planned.

I have a table with the following columns.

Column | Type | Modifiers | Storage | Description
--------+---------+-----------+----------+-------------
sat | text | not null | extended |
ts | bigint | not null | plain |
apid | integer | not null | plain |
bin | integer | not null | plain |
value | bigint | not null | plain |

A unique index on (sat, ts, apid, bin).

There are only a handful of unique sat values but there are about 20
million rows in the table as there are many apid values per unit time.

This query is fast and uses the index:

select max(ts)
from table
where sat = 'XX';

While this query results in sequential scans and long execution times:

select sat, max(ts)
from histograms
where sat in ('A1', 'A2', 'S1', 'S2')
group by 1;

Is there any way to formulate this query to make it faster without
adding an additional index?

Thank you in advance,
Wayne