The 2SC5994 is an interesting find. Comparing the DC Safe Operating Area curves of both devices, the 2SC5994 looks nearly an order of magnitude better than the 2SC4672.
Look at the 800 mA line on the left axis, follow this horizontally to the DC curve intersection, then down to the C-E voltage axis, for both devices. These graphs reveal the 2SC4672 really needs high beta and sufficient base current to keep the C-E junction well saturated, at 800 mA of collector current. Insufficient base current or beta can result in higher C-E saturation voltage, which pushes collector dissipation closer toward the DC SOA limit line.
The product of collector current and C-E saturation voltage is what determines the power dissipated, when the relay coil is pulled in continuously. There isn't much headroom for the 2SC4672. Note how much more margin exists for the 2SC5994, at the same 800 mA collector current line. This is about 1 kilometer wide, relatively speaking. Good choice.
RF+ Tech