I don't think that playing 4
♠ as a suggestion to play is so bad. But I read the auction a little different. The explanation of the auction that the original poster gave is from the East perspective. I will give you my interpretation of the auction:
1
♥-1
♠
2
♦-3
♣Fourth suit, can be bid for a number of reasons
3
♠Not my personal choice (3
♦), but I can understand it.
4
♦Suggests diamonds as a trump suit with an alternative strain available
After all, if East wanted to set diamonds as trump (without an alternative strain), he wouldn't have bid 3
♣. He would have bid 4
♦ immediately. Thus, the interpretation of the auction as given by he original poster seems inaccurate (unless they are playing an unusual version of fourth suit forcing e.g. with an immediate jump to 4
♦ giving west a choice between diamonds and spades).
4
♠The alternative strain is most likely spades. Let's play 4
♠ in a 5-2 fit.
I can also imagine that West thought that 3
♣ asked for a club stopper or spade tolerance. In that case, 4
♦ could easily be construed as a cue for spades. With that reasoning, 4
♠ is definitely the obvious bid.
I think that in the actual auction the 4
♦ bid was the culprit. If East had bid:
1
♥-1
♠
2
♦-3
♣
3
♠-3 NT
He would have shown that he wanted to play NT, with another strain possible. What could that strain be? It is not spades, since West gave East good news about spades. Hearts and clubs are unlikely too, which leaves us with diamonds.
What reason can East have to suggest a diamond contract when he has the values and the stoppers for 3NT? East must have some interest in 6
♦ (not 5
♦). Therefore, East also has some interest in 6NT.
In my opinion, this is a case of confusion about one of the most difficult conventions in bridge: Fourth suit forcing.
Rik
I want my opponents to leave my table with a smile on their face and without matchpoints on their score card - in that order.
The most exciting phrase to hear in science, the one that heralds the new discoveries, is not Eureka! (I found it!), but Thats funny
Isaac Asimov
The only reason God did not put "Thou shalt mind thine own business" in the Ten Commandments was that He thought that it was too obvious to need stating. - Kenberg
1♥ - 1♠
2♦ - 3♣
3♠ - 4♦
4♠ - 4NT
5♦ - 5NT
6♦ - Pass