We are concerned with the following nonlocal problem involving critical Sobolev exponent
−
a
−
b
∫
Ω
∇
u
2
d
x
Δ
u
=
λ
u
q
−
2
u
+
δ
u
2
u
,
x
∈
Ω
,
u
=
0
,
x
∈
∂
Ω
,
where
Ω
is a smooth bounded domain in
ℝ
4
,
a
,
b
>
0
,
1
<
q
<
2
,
δ
, and
λ
are positive parameters. We prove the existence of two positive solutions and obtain uniform estimates of extremal values for the problem. Moreover, the blow-up and the asymptotic behavior of these solutions are also discussed when
b
↘
0
and
δ
↘
0
. In the proofs, we apply variational methods.