The orbital period behaviours of four Algol-type eclipsing binaries SW Cyg, UU Leo, XX Cep and BO Vul are studied, by using all available times of minimum light in the literature. Their O-C diagrams were represented by long-period sinusoidal variations superimposed on parabolic forms. The parabolic forms for SW Cyg and UU Leo correspond to secular period increases, while for XX Cep and 130 Vul to secular period decreases. To explain these observed secular changes in these four Algol systems, we considered the combined effect of the mass transfer from the less massive component to the more massive one and the mass loss from the system. We concluded that the dominant mechanism for observed long-term period change of SW Cyg and UU Leo is the mass transfer, and that of XX Cep and BO Vul is the mass loss from the systems. It is interesting that the mass transfer rates (i.e., 10-(7)-10(-8) M-circle dot/yr) found for all four Algol binaries are at the upper limit of those generally accepted for Algols. However, assuming the mass loss via circumbinary disks of Chen et al. (2006) reduces the rate to 10(-8)-10(-9) M-circle dot/yr. We interpreted the tilted sinusoidal variations in all cases in terms of the light-time effect due to unseen components in the systems. Unacceptably large hypothetical third body masses in the case of SW Cyg and UU Leo, and common nature of the cyclic O-C variations in semi-detached Algols recall the cyclic magnetic activity effects of the secondary components as the working hypothesis in explaining cyclic period variations of these systems. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.