Ok, here are your options:
-Buy or make a PWM to 4-20mA converter. I am not sure if there are any commercially available ones, and the price may be the same as the VMA420.
-Use the VMA420 and feed it with a filtered output from your arduino.
-Use a short time-base PWM to drive a simple zero-crossing SSR. (I say short because you mentioned up to 50 seconds. This is quite long in terms of heating control, and a virtual eternity in terms of microcontrolers.)
The last one is what I would do for a heating process. If 3 seconds on will burn the product, it is no problem to turn on the SSR for a fraction of a second. In fact, I would set the time base at 3 seconds!