Can someone please explain to me why the mark of a triplet in this 4 half notes? I understood how to play it but from what i read you can only write triplets on 3 notes like 3 eighths 3 quarters etc... See picture
The problem is, that tuplet just means some note length modification takes place. Mathematically you require a standard fraction with numerator and denominator and there seem to be different opinions which of those to put into the tuplet bracket.
Wikipedia suggest under Tuplet Notation a full fraction representation in the form 2:3 for exotic cases, but I have never seen one.
So counting the note values and looking at the current time signature may be necessary for resolution.
To answer your last question: the use of 'triplet' indicator is allowable for any length note. It tells you to play the three marked notes so that each takes up 1/3 the meter-time that would have normally been covered by two of the notes in question. Thus, in quarter time meter, a one-beat triplet is written with eighth notes (normally two per quarter note), a two-beat triplet is written with quarter notes, etc.
The use of anything other than 2,4,8,16 as the denominator of the designated meter is highly discouraged, especially among those of us who have to perform the piece.