IFERR? Interception of an input error

[Onieh]

Hello,
I would not want which falls that programme if I ENTER presses without giving before 4 letters.
for example, EDUS or EDAX etc.
Even if I figures give the programme should not fall, sonder as long as wait to me 4 letters has given.

Addition: if I enter four letters and the is not there, the program should not crash, but the MSGBOX notify that the letters are not in the spreadsheet! i.e. ABCD

EXPORT Navigation()
BEGIN
LOCAL I,A,B,C,D,E,F,G;
LOCAL H;

STARTAPP("Flugplatz");

INPUT({{H,[2]}},"Flugplatzkennung"); //INPUT Airfield i.e. EDUS

RECT_P(0,0,320,240,RGB(0,205,205));

I:=POS(a:a,H);

TEXTOUT_P("F l u g p l a t z",90,0,7);

RECT_P(0,32,320,34);

TEXTOUT_P((EXPR("A"+I)),90,40,4);
TEXTOUT_P((EXPR("B"+I)),90,70,4);
TEXTOUT_P((EXPR("C"+I)),90,100,4);
TEXTOUT_P((EXPR("D"+I)),90,130,4);
TEXTOUT_P((EXPR("E"+I)),90,160,4);

WAIT();
FREEZE;

STARTVIEW (-1);
END;