To add to Dave's answer:
The 4kW rating is correct if you add the two 50Ω 2kW resistors in parallel, just be aware that the combined parallel resistance will now be 25Ω, not 50Ω. This means assuming your source voltage remains the same, your load will draw twice as much current (if you want a 4kW 50Ω load, you need 2 * 2kW 100Ω resistors in parallel)