The field on this nameplate is a shunt, not a series field. That simply means that it is wired as a separate circuit from the armature. Probably what jrwb4gbm means is that the field windings are split into two separate sections, each rated 4amps and 150V. You can wire them in parallel to create an 8amp 150VDC field or in series (as the OP indicates has been done) to create a 4amp 300V field.
It is the field current that is important on a DC motor, not so much the voltage. When a field winding is cold, its resistance is lower and it takes less voltage to get the rated current to flow. For this reason, DC drives usually provide a current regulated field supply.
The two field currents (4.0 and 2.4amps) listed in each field configuration are first, for the full rated field which gives you the full rated torque over the speed range of zero to base speed (1750rpm, in this case). If you weaken the field strength by reducing the field current, you reduce the available torque but increase the max speed in the same proportion. In this case, you can weaken the field down to 2.4amps which will cause the speed range to shift from zero to 2400rpm. The torque will be reduced from nominal by the ratio 1750/2400. Further reductions in field current are not advised since the motor can become unstable and may also exceed its maximum rated speed.
It is very important on DC motors to observe the maximum speed limit. DC motors have wound armatures and, compared to AC motors, they are very fragile. On this motor, I would take the 2400rpm to be the maximum safe speed and it should be considered the absolute max limit.
The OP didn't say, but the nameplate should also say something about the way the field is built. The choices are: straight shunt, stab. shunt, and comp shunt. The straight shunt motor only has the shunt field we described above. The other two are compound wound fields having the shunt field plus an additional series field that is part of the armature circuit.
A compound field motor is not symetrical as to direction. It makes more torque forward than reverse. It is essential that the series field be properly wired (the S1 and S2 leads) or the motor will behave very poorly. A straight shunt motor will not have S1 and S2 leads.
Hope this clarifies a few things.