The key of E major and its relative minor, c sharp minor, both have four sharps. Additionally, key signatures apply to all music not just the piano and remain the same regardless of instrument. When referring to the piano the term "key" usually refers to on of the actual white or black keys that are pressed down to make a sound of the corresponding pitch. Each key on the piano is attached to a hammer that hits a string inside the instrument, thus the piano's classification as a percussion instrument. The strings are of different lengths and thicknesses, creating the different pitches. Each key on the piano is responsible for one pitch. Therefore a single key on the piano cannot "have four sharps", as you asked.
D major has F# and C# in it's key signature.
A sharp key signature is one that contains sharps.
There are no sharps in the key signature, but in the harmonic minor there is a G#.
There's no sharps of flats in the key signature in either C major or A minor.
What is a key signature? A key signature is the number of sharps or flats in a piece of music. For example C Major has no sharps G Major has 1 Sharp F Major has 1 Flat B flat
The scale with four sharps in its key signature is the E major scale.
The key of C major has the most sharps in its key signature, with seven sharps.
The key signature with 3 sharps is A major.
The key signature with three sharps is A major.
The key with 5 sharps in its key signature is the key of B major.
The key with five sharps in its key signature is the key of B major.
The key signature with 5 sharps is D major.
The major key with 5 sharps in its key signature is B major.
The major key with 5 sharps in its key signature is B major.
The key signature with 5 sharps in music is the key of B major.
The key signature that contains 5 sharps is the key of B major.
The key signature that contains 7 sharps is the key of C major.