4) Dirty bomb - conventional explosive technology, using radioactive materials in bomb casing, to be spread by the explosive. Very constructable by terrorists, if they are provided with radioactive material. And not requiring the fissile material. Nuclear "waste" material would serve.
Neither of the last two will have the impact of the first two, as they are not atomic explosions, but a criticality device can kill a number of people, particularly if properly planted, as it could be constructed to emit only radiation, and thus be virtually undetectable without radiation detection instrumentation.
A dirty bomb could contaminate a multi block area, possibly with dangerous levels of hazardous materials, requiring an intensive, and expensive decon effort, but nothing that is technically difficult.