EW fail to end up in the best fit — to put it mildly...

N/EW
♠	K Q J 10 6 5			♠	A 8 3
♥	2			♥	J 10 9 8
♦	8 7 6			♦	A 3 2
♣	Q 8 4			♣	K J 9

¹ Multicoloured; usually weak two in a major

East didn't make 4♥. The trump suit in particular gave him some problems...
West: 'I thought that you wanted to play 4♥.'
East: 'I wanted to offer you a choice between 4♥ and 4♠.'
Who erred?

Solution

Assuming that EW have not (!) made special agreements about responding to partner's Multicoloured 2♦ opening, West was wrong.
Without special agreements 4♥ says: 'I want to play 4♥ if hearts is your six-card suit; but you should bid 4♠ if your six-card suit is spades.' This is known as: pass or correct.
The idea is to be able to raise the bidding level as quickly as possible. NS often have no idea now, whether East is weak (in which case he is bidding pre-emptively) or strong (in which case he bids in order to make).
If East has a long suit and aims to play 4♥, he begins by bidding 2NT (forcing relay). West now describes his hand. Next East bids 4♥. He knows now that West has spades (and West knows that East knows!), so 4♥ is to play.

For advanced players***.
Nowadays many advanced partnerships have made special agreements about responding to partner's Multi. We'll take a look at these agreements.
If the partner of the Multi opener has at least 3-3 in the majors (so a nine-card fit is certain) and decides that the partnership should bid four in the major fit (either as a pre-emptive bid or to make), these advanced methods offer two possible approaches. The idea is that the partnership should be able to choose which hand is to become declarer.

¹ Multi
² We are going to play at the four level with your six-card suit as trumps, but I should be declarer; therefore I want you to make a transfer bid to your major six-card suit
³ Six-card heart suit
⁴ This is what South wanted from the start: he is now the declarer, while his partner's major suit is trumps; South will choose this approach if he has a lot of tenaces: he will get the lead running up to his hand

If, in the bidding above, North would have had six spades, he would have bid 4♥ over 4♣, over which South would have bid 4♠.

¹ Multi
² We are going to play at the four level with your six-card suit as trumps,  but you should be declarer; therefore I want you to bid your six-card suit
³ With six spades North would have bid 4♠

Let's return to the original bidding problem.
1. If EW have agreed upon the advanced methods, East's 4♥ bid is to play, so West is right then and East wrong. Yes, conventions (like the Multi) are useful, but only if a partnership has made thorough agreements about the follow-up. And that's where things often go wrong, a major source of irritation...
2. Partnerships that have not agreed upon these advanced agreements play East's immediate 4♥ bid as pass or correct, as we have seen. However, with the given East hand that seems to be a bit too aggressive. That shows: if West had correctly corrected by bidding 4♠, he wouldn't have had a chance: down one would have been the sad result.
Probably East had better bid 3♠. Such a bid in a major below game level is always pass or correct:
- With six spades (EW have only a 6-3 trump fit then) and a minimum, West will pass.
- With six hearts West has to correct to 4♥. No problem, since EW then have a 6-4 trump fit, meaning East wants to be in game, even if West has a minimum.
If East is a real wise guy, he will probably consider a 3NT contract. That will succeed here if NS fail to attack in diamonds in time.