Nonlinear optical (NLO) properties of the colloidal solutions of chemically synthesized undoped and Mn2+ doped ZnS Quantum Dots (QDs) in methanol are measured by using a Q-switched 10 ns pulsed Nd∶YAG laser radiation by the Z-scan technique. The nanostructures of the synthesized materials are characterized by using different characterization tools, such as Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD) analysis. Linear optical absorption and Photoluminescence (PL) emission characteristics of the colloidal solutions of the synthesized QDs are measured at room temperature by using a UV-visible spectrophotometer and a spectrofluorimeter, respectively. The absorption characteristics of the samples show that the absorption cut-off of the samples is below that of the bulk ZnS due to the quantum confinement effect. Photoluminescence emission characteristics measured at room temperature show that the Mn2+ doped ZnS sample exhibits its visible PL emission peak at ~580 nm, whereas the undoped ZnS sample emits in the ultraviolet region peak at ~365 nm. The average particle size (radius) of the as-prepared ZnS sample is ~1.2 nm as determined from the measured UV-visible absorption characteristics as well as from TEM and XRD data analyses. By analyzing the experimental data obtained by the Open Aperture (OA) Z-scan technique, it is found that the Four-photon Absorption (FPA) takes place at 1064 nm wavelength in both the studied samples. FPA coefficients and FPA cross-section of both the samples are extracted by fitting the experimental data with the available analytical expression. It is found that the calculated value of FPA cross section of ZnS QD is 4.9×10-106 cm8·s3·photon-3, which is five orders of magnitude larger than that of bulk ZnS. Optical limiting property of the synthesized ZnS QD is also presented. The simultaneous presence of large FPA cross section and large luminescence efficiency in the visible region in Mn2+ doped sample would render this material as a good candidate for multiphoton fluorescence imaging applications.