A guided-ion beam tandem mass spectrometer is used to study the reactions, W+ + CH4 (CD4) and [W,C,2H](+) + H-2 (D-2), to probe the [W,C,4H](+) potential energy surface. The reaction W+ + CH4 produces [W,C,2H](+) in the only low-energy process. The analogous reaction in the CD4 system exhibits a cross section with strong differences at the lowest energies caused by zero-point energy differences, demonstrating that this reaction is slightly exothermic for CH4 and slightly endothermic for CD4. The [W,C,2H](+) product ion reacts further at thermal energies with CH4 to Produce W(CH2)(x)(+) (x = 2-4). At higher energies, the W+ + CH4 reaction forms WH+ as the dominant ionic product with smaller amounts of WCH3+, WCH+, and WC+ also formed. The energy dependent cross sections for endothermic formation of the various products are analyzed and allow the determination of D-0(W+-CH3) similar to 2.31 +/- 0.10 eV, D-0(W+-CH2) = 4.74 +/- 0.03 eV, D-0(W+-CH) = 6.01 +/- 0.28 eV, and D-0(W+-C) = 4.96 +/- 0.22 eV. We also examine the reverse reaction, [W,C,2H](+) + H-2 (D-2) -> W+ + CH4 (CH2D2). Combining the cross sections for the forward and reverse processes yields an equilibrium constant from which D-0(W+-CH2) = 4.72 +/- 0.04 eV is derived. Theoretical calculations performed at the B3LYP/HW+/6-311++G(3df,3p) level yield thermochemistry in reasonable agreement with experiment. These calculations help identify the structures and electronic states of the species involved and characterize the potential energy surface for the [W,C,4H](+) system.