Please note: All of the following is just wishful goddam thinking, and Grimiku has made an official statement that the whole "stash tab for Season Journey is as FUBAR as everyone has been thinking it is...now it's official!
Kevin Martens, the Diablo III Lead Designer appears in the following YouTube BlizzCon video beginning at the time linked below:
He specifically begins talking about the Stash tabs situation at 47:40 in that video, linked below:
The following is a transcript of that segment, which if interpreted in the sense that it should be, based on the inaccuracies of human speech, then it should settle the issue about when Stash tabs are issued, and what, finally, the Season Journey is.
I think were going to give you, right at the beginning of patch 2.4, as soon as it launches, if you go to your Stash there's a new button there… For normal in game gold amount, you can immediately buy an extra tab.
[Unknown orator] You think?
All right, so, one tab, that's cool.
I notice there's a lot of room for additional tabs there...
[Apparently a video edit here]
Furthermore in Season Journey, you will get a new Stash tab every Season Journey that you play from now on, up to... Hold on…hahahaha... Wait for it...
[Apparently a video edit here]
Right...uh... Let's take a look at that real quick, so here it is, in Season five, you have one tab… You finish, in this case, the Conqueror objective for Season five, and automatically, no in game gold cost, totally free, another tab.
This will continue up to a maximum of 10 tabs, that is to say, doubling your current stash space.
[Video edit to audience applause here]
Because I thought this might be popular, I'm going to re-summarize that one more time in a slide in case people want to take pictures… "Wait, how do I get these again?" All right.
Everyone gets a new tab immediately on 2-4, in game gold cost, rather moderate, 500,000 gold.
Every Season Journey, starting with the first Season - so you get two stash tabs right off the bat. If you complete the objective in the Season Journey you'll get more, to a total of 10.
[Here's a link to a picture of the slide:
This is all very PC oriented, so console players out there…
[And he continues talking about console issues.]
Some clarification here.
First of all, above when he says, "Every Season Journey, starting with the first Season…" What he means is the first Season after Patch 2.4 deploys, which would be Season 5.
Secondly, from Season 5 on, for every Season Journey you complete, in the instance he was using, Conqueror, a very poor and confusing example in my opinion, on completion of that Season Journey…each and every Season Journey… you are awarded a Stash tab, up to 10 total.
That means, in Season 5, you can immediately buy one tab for gold, and earn 4 more through the Season Journeys during that Season.
Everyone who plays Season 5 should have 10 stash tabs at the end of the Season.
What the Heck Is a Season Journey Anyway?
When you log into the game, look at the right-hand side of the "Start Game" page. Note where it says:
x/x Objectives Complete
Click on the View Progress button.
In the top center of the "Season Journey" dialog, there is a drop-down list box. Click the down arrow.
Scroll down to Chapter I and click it.
Look at the right-hand pane of the Season Journey dialog.
Note that it says, very plainly:
And so on and so forth...
Each of those represents a "Season Journey," within a single Season.
Which means all players can have all ten stash tabs in a single Season, the first Season after Patch 2.4 is deployed, Season 5.