Results of broadband dielectric spectroscopy (BDS) studies, for a remarkable temperature range (130 K), in supercooled rodlike liquid crystalline n-butylcyanobiphenyl (4CB) are shown. They are supplemented by static nonlinear dielectric effect (NDE) measurements in 4CB and 3CB (n-propylcyanobiphenyl), giving unequivocal estimations of the hypothetical isotropic-nematic (I-N) continuous phase transition temperature T* and of the discontinuity of this transition deltaT. The distribution of relaxation times becomes strongly non-Debye on approaching the I-N transition and follows the Debye pattern for T > T(I-N) + 75 K. The temperature evolution of relaxation times [tau(T)] and dc conductivity [sigma(T)] remain non-Arrhenius in the tested temperature range. Their descriptions via the Vogel-Fulcher-Tammann (VFT), Cohen-Grest (CG), and mode coupling theory (MCT) based relations are discussed. The derivative analysis of tau(T) and sigma(T) experimental data enabled the detection of dynamic crossovers and the insight into the behavior of the apparent activation energy which follows the relations E(A)(tau),E(A)(sigma) (sigma) is proportional to (T/T-T0)2, with a small distortion in the immediate vicinity of the I-N transition. The glassy dynamics coexist with the critical-like behavior of temperature dependences of the static dielectric permittivity, maximum of the loss curves and strong pretransitional NDE pretransitional anomaly. These results indicate the significant role of prenematic fluctuations-heterogeneities in the isotropic, fluidlike, surrounding regarding both static and dynamic properties.