Bicyclo[1.1.0]but-1(2)-ene (BBE), one of the smallest bridgehead alkenes and C4H4 isomers, exists theoretically as a reactive intermediate, but has not been observed experimentally. Here we successfully synthesize the silicon analogue of BBE, tetrasilabicyclo[1.1.0]but-1(2)-ene (Si4BBE), in a base-stabilized form. The results of X-ray diffraction analysis and theoretical study indicate that Si4BBE predominantly exists as a zwitterionic structure involving a tetrasilahomocyclopropenylium cation and a silyl anion rather than a bicyclic structure with a localized highly strained double bond. The reaction of base-stabilized Si4BBE with triphenylborane affords the [2+2] cycloadduct of Si4BBE and the dimer of an isomer of Si4BBE, tetrasilabicyclo[1.1.0]butan-2-ylidene (Si4BBY). The facile isomerization between Si4BBE and Si4BBY is supported by theoretical calculations and trapping reactions. Structure and properties of a heavy analogue of the smallest bridgehead alkene are disclosed.