To analyze which statements are true regarding the effects of short-time duty on the size and cost of electric motors, we need to understand the concept of short-time duty in electrical machines. Short-time duty refers to operating a motor for a short period at its full rated capacity, followed by sufficient time for the motor to cool down.

1. Statement (i): Short-time duty allows use of higher rated motors for short durations.
   This statement is true. In short-time duty, motors can be used for higher power ratings because they operate for short periods and do not have to dissipate continuous heat over an extended period. This allows for the use of motors at higher ratings than they could manage continuously.
2. Statement (ii): Motors used for short-time duty are generally smaller for the same load.
   This statement is also true. Since the motor operates only for a short period, it can be designed smaller because the heat produced during operation does not reach the levels that would occur in continuous operation. Thus, motors are typically smaller for short-time duty when designed for the same load.
3. Statement (iii): Short-time duty increases the need for oversized enclosures.
   This statement is false. Since the motor size can be reduced due to the shorter operating time, there is no requirement for oversized enclosures.
4. Statement (iv): Cost per kW is lower for motors used in short-time duty.
   This statement is true. Short-time duty motors are less expensive per kW of capacity because they require less material for construction due to their smaller size and intermittent operation.

Given these analyses, the statements that are true are (i), (ii), and (iv). Therefore, the correct answer is (i), (ii), and (iv) only.