It looks like this problem is leading up to the fundamental theorem of Galois theory, which states that there is a one-to-one correspondence between the subfields of a Galois extension and subgroups of its Galois group. So our answer will be, of course, that no such Galois extension exists (which is why it's difficult to find an example).
First, note that if $K$ is a Galois extension of degree $4$ over $\mathbb{Q}$, then it must be the splitting field of a quartic polynomial. Either that polynomial is irreducible, or it can break into a product of irreducible quadratics. Of course, you'll want convince yourself of these cases and that the cases are indeed exhaustive.
If the polynomial breaks into a product of two irreducible quadratics, then I claim that it's rather easy to show that $K$ must have a subfield.
On the other hand, if the polynomial is irreducible, then $K = \mathbb{Q}[\alpha]$, where $\alpha$ is one of its roots. Since the degree of the Galois extension is $4$, we also know the Galois group has order $4$. As such, the Galois group must have an element $\phi$ of order $2$ that sends $\alpha \mapsto \beta$, where $\beta$ is another root of the polynomial.
Claim: $\mathbb{Q}[\alpha\beta] \subsetneq \mathbb{Q}[\alpha]$
To show this, suppose for contradiction they were actually the same field. Then both would be of degree $4$ over $\mathbb{Q}$, and the former would have a basis $\{1, \alpha\beta, (\alpha\beta)^2, (\alpha\beta)^3\}$. Because $\alpha$ would be an element of both fields, we could write:
$$\alpha = c_1 + c_2(\alpha\beta) + c_3(\alpha\beta)^2 + c_4(\alpha\beta)^3$$
What happens when the automorphism $\phi$ acts on both sides of this expression? What can we conclude?
Best Answer
Be sure you can follow the following providing justifications:
First of all, what can be $\;n:=\;$ the extension's degree? Clearly $\;n\;$ can't be a prime number (why?), and it can't also be that $\;n\;$ has more than two proper non-trivial divisors. Thus, we either have $\;n=pq\;$ , for two different primes $\;p,q\;$ , or else $\;n=p^3\;$ for some prime $\;p\;$ .
In the first first case $\;n=pq\;$: if the extension isn't cyclic then the Galois Group isn't even abelian and is thus the non-trivial semidirect product of two cyclic groups of order $\;p,\,q\;$. This case is possible only if $\;p<q\;$ and $\;p\,\mid\,(q-1)\;$ , say $\;q-1=kp\;$ , which would then give us $\;q>1\;$ different subgroups of order $\;p\;$, a contradiction. Thus, $\;|G|=pq\;$ is the cyclic group of order $\;pq\;$ and indeed has one single subgroup of order $\;p,\,q\;$, and both subgroups correspond with normal extensions of $\;F\;$ since the whole extension is abelian and thus the subgps. are abelian, too.
If $\;[K:F]=|G=\text{Gal}\,(K/F)|=p^3\;$ , the group $\;G\;$ has two subgroups of orders $\;p,\,p^2\;$ . If $\;G\;$ isn't cyclic then it must have at least one subgroup more of at least one of the two orders $\;p,\,p^2\;$, which cannot be true by assumption...