One motor driver to control 4 DC motors (Current effect)

I am going to use one L298 motor driver to control 4 DC motors, I was wondering will that affect the speed of the motors due to the current load?

Yes, current load and voltage drop across the L298 output.
Voltage drop there is equal to power dissipated as heat in the device,
vs that same power (current x voltage) being available for your motor.

New MOSFET parts have much less voltage drop across them, thus more power is available to your motor.
Check the motor drivers here, might even find something on sale you can use during their Pi day sale thru tomorrow.

Thanks for the answer, pretty helpful.

You ask a question without providing any information on the specs of the motors. How can that work?

Yes sorry, we haven't chosen the motor yet but mostly it will be something with specs near that.

Gear ratio: 100:1
rpm @ 6V: 120
Rated current: 30mA @ 6V
Stall current @ 6V: 420mA
Rated torque @ 6V: 150
Stall torque @ 6V: 540

So 0.42A stall current per motor, therefore 1.7A total, which will be fine with 2 motors per channel
with the L298 (driving them all from one channel might be pushing it though, but is feasible)

The L298 will lose 2 to 2.5V anyway, being darlington-based, so you probably want to consider > 6V for
the supply, 8 or 9V, with 2A or more capability.