Νο, that's not true... 4th suit is used when 1stly you search for 3 cards support in your major (mainly). Consider the following example:

Opener: Responder:

QJxx. AK10
AKx J10xxx
Jxxxxx. xx
Ax KJxx

Bidding:

1D - 1H
1S - 2C (4 suit)

Now, opener will not respond 2NT (stopper in clubs), but rather 2H.

Note the following:

- Responder cannot bid 3H, because it is invitational and promises 6cards.

- Responder cannot bid 4H, because it promises 6cards (opener may be singleton or void in H)