Three equatomic TaHfZrTi, NbHfZrTi and TaNbHfZrTi refractory high entropy alloys (HEAs) were synthesized by arc-melting and subjected to solution treatment, followed by three cooling conditions, namely, furnace cooling (FC), air cooling (AC) and water quenching (WQ). The microstructure, phase constitution and mechanical properties of these heat treated RHEAs were characterized. Surprisingly, strong dependence of microstructures on cooling rates was revealed in the TaHfZrTi and TaNbHfZrTi, but not in the NbHfZrTi. The TaHfZrTi remains a BCC (body centered cubic) lattice in WQ samples and decomposes into two BCC phases with different lattice parameters in AC samples with a slower cooling rate. The phase decomposition in the FC TaHfZrTi is more complete; BCC Ta-rich and hexagonal-close-packed (HCP) Ti-rich precipitates form in the matrix BCC. Our results indicate that processing conditions in some HEAs like TaHfZrTi needs to be carefully controlled as far as the structural stability is considered.