Question : COUNT_variable Error

I to have tblModellTmp with HP_4 values which represent the Sector for each company. I would like now to count how many companies are in each Sector and then in a second step, save this value for each company in HP_5 so I know for each how many competitors around. But I get already an error in my first line saying the subquery has gave back more then one value

SET @HP_4_Count = (SELECT COUNT(HP_4)FROM tblModellTmp WHERE (T = 1)GROUP BY HP_4)

UPDATE tblModellTmp
SET HP_5 = @HP_4_Count

the simble query
SELECT     COUNT(HP_4) AS Expr1, HP_4
FROM         tblModellTmp
WHERE     (T = 1)
GROUP BY HP_4

gives back
Expr1      HP_4
78      Elektrogeräte & Bauteile
72      Freizeit, Unterhaltung, Hobbies
42      Möbel, Hausrat, Haushaltgeräte
...

Any idea how to solve?
thx
Kongta

Answer : COUNT_variable Error

sorry

UPDATE u
SET HP_5 = x.expr1
from tblmodelltmp as U
inner join (
SELECT     COUNT(HP_4) AS Expr1, HP_4
FROM         tblModellTmp
WHERE     (T = 1)
GROUP BY HP_4
) as x
on u.hp_4=x.hp_4
where u.hp_5 <> x.expr1