Effective mass of the four-flux composite fermion at nu=1/4

We have measured the effective mass (m*) of the four flux composite fermion at Landau-level filling factor v= 1/4 ((CF)-C-4), using the activation ene rgy gaps at the fractional quantum Hall effect states v= 2/7, 3/11, and 4/1 5 and the temperature dependence of the Shubnikov-de. Haas (SdH) oscillatio ns around v= 1/4. We find that the energy gaps show a linear dependence on the:effective magnetic field B-eff (= B-B-v=1/4), and from this linear depe ndence we obtain m(*)= 1.0m(e) and a disorder broadening Gamma similar to 1 K for a sample of density n = 0.87 X 10(11) cm(-2). The m* deduced from th e temperature dependence of the SdH effect shows large differences for v> 1 /4 and v< 1/4. For v>1 /4, m* similar to 1.0m(e). It scales as root B-v wit h the mass derived from the data around v= 1/2 and shows an increase in m* as y--> 1/4, resembling the findings around v= 1/2. For v< 1/4, m* increase s rapidly with increasing B-eff and can be described by m*/m(e) = - 3.3+ 5. 7(eff). This anomalous dependence on B-eff is precursory to the formation o f the insulating phase at still lower filling.