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We present JHK and 3.8-micron photometry of 26 galaxies in the Infrared SpaceObservatory (ISO) Normal Galaxy Key Project (KP) sample and of seven normalellipticals with the aim of investigating the origin of the 4-micron emission.The majority of the KP galaxies, and all the ellipticals, have K-L<~1.0,consistent with stellar photospheres plus moderate dust extinction. Ten of the26 KP galaxies have K-L>~1.0, corresponding to a flat or rising 4-microncontinuum, consistent with significant emission from hot dust at 600-1000K. K-Lis anticorrelated with ISO flux ratio F_{6.75}/F_{15}, weakly correlated withline ratio [OI]/[CII], but not with [CII]/FIR or IRAS ratio F_{60}/F_{100}.Photodissociation-region models for these galaxies show that the hot dustresponsible for red K-L resides in regions of high pressure and intensefar-ultraviolet radiation field. Taken together, these results suggest thatstar formation in normal star-forming galaxies can assume two basic forms: an``active'', relatively rare, mode characterized by hot dust, suppressedAromatic Features in Emission (AFEs), high pressure, and intense radiationfield; and the more common ``passive'' mode that occurs under more quiescentphysical conditions, with AFEs, and without hot dust. The occurrence of thesemodes appears to only weakly depend on the star-formation rate per unit area.Passive star formation over large scales makes up the bulk of star-formingactivity locally, while the ``active'' regime may dominate at high redshifts.Comment: 11 pages, 7 figures: A&A, in press (replaced 30/09/02 for error with  macro

Hot dust in normal star-forming galaxies: $\vec{JHK}$ $L^\prime$ photometry of the ISO Key Project sample