SQL & PL/SQL :: Pull Records With Specific Criteria

I want to start by saying I am brand new to SQL. I have an access database linked to my oracle and am trying to query a very specific set of data and I can't seem to narroe it down. I have 244,000 lines in the DB and I'm trying to find items on a specific trype of vendor agreement. I may have the same item on multiple agreements. ex 1-, 1a-, 2-,2a-,3-,3a-,4-,4a-,5-,5a-,6-,6a-,7-,7a-,8-,8a-.

each item by agreement is on it's own line.
ex.

item ven_agrmt_ref
233 1a-xxx
233 4-xxx
233 4a-xxx
255 4a-xxx

I need to find a way to select just items that appear on a 4- or 4a- and no other agreement reference. The query I did so far pulls all of the 4- and 4a- agreements but will also pull items,like #233 in the example above, but not showing the 1a- agreement. I need it to overlook that item eventhough it does appear on the agreement I am looking for but also has an agreement I am not. the statement I am using right now is:

SELECT item, ven_item, ven_agrmt_ref, base_cost, vendor
FROM "all items by agreement"
WHERE ven_agrmt_ref >= '4'
ORDER BY item