This is an example in Fulton and Harris's
*Representation Theory: A First Course* (Springer **GTM** 129 (1991)),
except that they work only with $SL(V)$, not $GL(V)$.
The representation you call $Ext\;V$ is denoted $\wedge^2 V$.
Its tensor product with $V$ has a natural map to $\wedge^3 V$,
which is isomorphic with the contragredient $V^*$ as a representation
of $SL(V)$, but not quite for $GL(V)$ (where $\wedge^3 V$ is the
tensor product of $V^*$ with the 1-dimensional determinant
representation). For the kernel of this map, Fulton and Harris
say on page 220 that it contains "the irreducible representation
$\Gamma_{1,1,0}$ with highest weight $2L_1+L_2$", and then Exercise 15.10
asserts that the kernel is in fact this irreducible representation.

So in summary Fulton and Harris say that $V \otimes \wedge^2 V$
is the direct sum of irreducible representations of dimensions $4$ and $20$,
the former of which is $\wedge^3 V = V^* \otimes \det$.

P.S. The Fulton-Harris text also shows pictures of
the root system etc. for $A_3$.