The number of pi-electrons in the molecules of aromatic compound is:

Concept: Huckel's rule states that a planar, cyclic, fully conjugated molecule is aromatic only if its ring contains (4n + 2) π-electrons, where n = 0, 1, 2, 3... This π-electron count is the defining criterion for aromaticity.

Application: For an aromatic compound, the delocalized π-electron cloud in the ring must satisfy this (4n + 2) count for the extra stability of aromatic delocalization to appear. For example, benzene has n = 1, giving 4(1) + 2 = 6 π-electrons. So the general formula for the number of π-electrons in aromatic molecules is 4n + 2.

• 4n electrons (n = 1, 2, 3...) is instead the criterion for ANTI-aromaticity, as in cyclobutadiene (4 π-electrons) -- this destabilizes a planar cyclic conjugated system rather than stabilizing it.

• 4n + 1 does not correspond to any recognized aromaticity or antiaromaticity electron-counting rule for cyclic conjugated systems.

• 2n + 4 is not a valid electron-counting expression used in aromaticity theory and does not track how the π-electron count actually scales across conjugated aromatic rings.

Therefore, the correct general count of π-electrons in an aromatic compound is 4n + 2, as given by Huckel's rule.