SELECT ROOM, 
case when 
p1<=p2 and p1<=p3 and p1<=p4 then p1
when 
p2<=p1 and p2<=p3 and p2<=p4 then p2
when 
p3<=p1 and p3<=p2 and p3<=p4 then p3
when 
p4<=p1 and p4<=p2 and p4<=p3 then p4
end p
 FROM Table

select room, min(p) p from
(
SELECT ROOM, 
case when 
p1<=p2 and p1<=p3 and p1<=p4 then p1
when 
p2<=p1 and p2<=p3 and p2<=p4 then p2
when 
p3<=p1 and p3<=p2 and p3<=p4 then p3
when 
p4<=p1 and p4<=p2 and p4<=p3 then p4
end 
p
 FROM Table
) subq group by room 

SELECT Table.ROOM, 
Min(IIf(p1<=p2 And p1<=p3 And p1<=p4,p1,
IIf(p2<=p1 And p2<=p3 And p2<=p4,p2,
IIf(p3<=p2 And p3<=p1 And p3<=p4,p3,
IIf(p4<=p2 And p4<=p3 And p4<=p1,p4))))) AS p
FROM [Table]
GROUP BY Table.ROOM;

You tried to execute a query that does not include the specified expression 
'IIf(.... бла-бла-бла ... ))))' as part of an aggregate function