FOC speed limit motor?
You have two current controlers in your FOC Iq and Id. If you drive a PMSM motor, then Id has to be 0, and Iq is the one which controlls the torque. 

It is obvious if you limit the Iq to 4A, the motor will accelerate until it will deliver a setpoint torque which is related to the 4A setpoint.

Now if there is no load presen and the motor spins at high revs, the controller can't inject more current because the back EMF is now the same as supply voltage. This BEMF is related to the motor constant.

EDIT:

d-axis stand for excitation, menawhile q- axis for torque. It is similar to brush DC motor with separate excitation and with permanent magnet. Iq is the armature current, Id is the field current. Since you already have a permanent magnet, then Id shall be zero.



Vq and Vd are also similar to brush DC, except they are also cross linked with Iq and Id respectively.

$$u_d = R_s i_d+L_s\dfrac{di_d}{dt}-\omega L_s i_q  $$
$$u_q = R_s i_q+L_s \dfrac{di_q}{dt} +\omega (L_si_d+\lambda_{pm})$$

If the id=0 and it's kept constant, neglecting the resistance , then the vd has to be: $$ -\omega L_s i_q $$ That's the feedforward component that user JonRB refered at.
Meanwhile the vq can be simplified to the similar formula as brush DC motor armature voltage:

$$u_q = R_s i_q+L_s \dfrac{di_q}{dt} +\omega k_e$$