I can prove $(3) \Rightarrow (1)$ when $X$ and $Y$ are connected (**Lemma 1** below; no integrality assumptions needed), and I can prove $(4) \Rightarrow (1)$ when $X$ and $Y$ are normal, connected, and Noetherian (in particular integral, see [Tag 033M][6]) (**Lemma 2** below).

I do not know how to approach the general case if $Y$ is not normal; see the remark at the end. Maybe you can weaken it to something like unibranch? The property that I need is that a connected finite étale cover of $Y$ is always integral; I don't know in what generality this is true.

**Reference.** A good introductory reference to some of the stuff I'm using is the chapter *Fundamental groups of schemes* of the Stacks project. (I make precise references as I go along.)

**Notation.** Let $\eta_X$ (resp. $\eta_Y$) be the generic point of $X$ (resp. $Y$). I will use the following definition.

**Definition.** Let $\phi \colon X \to Y$ be a morphism of schemes, let $G$ be a finite group (viewed as constant étale group scheme), and let $G \times X \to X$ be an action of $Y$-schemes. Then $\phi$ is *Galois with group $G$* if the induced map $G \times X \to X \times_Y X$ is an isomorphism.

If $F \colon \operatorname{FÉt}_Y \to \operatorname{Set}$ is a fibre functor (notably, $F = F_{\bar y}$ associated to taking the geometric fibre over a geometric point $\{\bar y\} \to Y$), there is the following equivalent notion of being Galois.

> **Auxiliary lemma.** Let $\phi \colon X \to Y$ be a finite étale morphism of connected schemes. Then $|\!\operatorname{Aut}_Y(X)| \leq |F(X)|$, and equality holds if and only if $\phi$ is Galois.

*Proof.* Let $G = \operatorname{Aut}_Y(X)$, with the natural action on $X$, and consider the induced map
$$\psi \colon G \times X \to X \times_Y X.$$
Applying the fibre functor $F = F_{\bar y}$ gives
\begin{align*}
F(\psi) \colon G \times F(X) &\to F(X) \times F(X)\\
(\sigma, s) &\mapsto (\sigma(s),s).
\end{align*}
By [Tag 0BN0][1] (7), this map is injective, and it is bijective if and only if $|\!\operatorname{Aut}_Y(X)| = |F(X)|$.

Thus we need to show that $\psi$ is an isomorphism if and only if $F(\psi)$ is a bijection. This follows since $F$ reflects isomorphisms (see [Tag 0BMY][2] and [Tag 0BNB][3]). $\square$

--------

> **Lemma 1.** Let $\phi \colon X \to Y$ be a finite étale morphism of connected schemes. Assume there exists a finite group $G$ and an action $G \times X \to X$ whose base change to $\{\eta_X\}$ makes $\{\eta_X\} \to \{\eta_Y\}$ a Galois extension with group $G$. Then $X \to Y$ is Galois with group $G$.

*Proof.* By the auxiliary lemma, we only have to show that $X$ has enough automorphisms. This is now a simple degree/counting argument (the elements of $G$ providing the automorphisms). $\square$

> **Lemma 2.** Let $\phi \colon X \to Y$ be a finite étale morphism of connected normal Noetherian schemes, and assume that the extension $\{\eta_X\} \to \{\eta_Y\}$ is Galois. Then $\phi$ is Galois.

*Proof.* We want to apply [Tag 0BN6][4] $(3) \Rightarrow (2)$ to the pullback functor
\begin{align*}
H \colon \operatorname{FÉt}_Y &\to \operatorname{FÉt}_{\eta_Y}\\
Z &\mapsto Z_{\eta_Y}.
\end{align*}
Assume $Z$ is a connected scheme finite étale over $Y$. Then $Z$ is normal ([Tag 025P][5]), connected, and Noetherian; hence integral ([Tag 033M][6]). Hence, it only has one generic point $\eta_Z$, and this is the only point lying over $\eta_Y$. Thus, $H(Z)$ is connected, so [Tag 0BN6][4] $(3) \Rightarrow (2)$ implies that $H$ is fully faithful.

Note that $H$ also reflects isomorphisms, since $F_{\overline{\eta_Y}}$ does, which factors through $H$. For a fully faithful functor that reflects isomorphisms, we have
$$\operatorname{Aut}_Y(X) = \operatorname{Aut}_{H(Y)}(H(X)),$$
so $X$ has enough automorphisms since $H(X)$ does so. $\square$

**Remark.** We see that $H$ is fully faithful if and only if for every connected étale $Y$-scheme $Z$, the generic fibre $Z_{\eta_Y}$ is just a point. This is certainly the case if $Z$ is integral, and I think the converse holds as well. (In fact, I'm not even sure if it's possible for $Z$ to have multiple irreducible components, so this might be a vacuous statement...)

Also, we might not need the full strength of being fully faithful: we only need to get back all *automorphisms*.


  [1]: http://stacks.math.columbia.edu/tag/0BN0
  [2]: http://stacks.math.columbia.edu/tag/0BMY
  [3]: http://stacks.math.columbia.edu/tag/0BNB
  [4]: http://stacks.math.columbia.edu/tag/0BN6
  [5]: http://stacks.math.columbia.edu/tag/025P
  [6]: http://stacks.math.columbia.edu/tag/033M