The same issue that caused the acceleration problem can also be partially responsible for wheel slips. During acceleration and normal running, when the reaction torque sets in and the front wheels rise up slightly, they become more prone to slipping and in such cases, the un-powered axle ( which in case of WDP4 is the one closest to the loco centre I guess) does not really contribute anything to mitigate the loss of tractive effort. And since the original WDP4 did not come with independent traction control for each TM, this could have potentially lead to wheel slip problems in slightly adverse conditions such as wet tracks or mild gradients. Most probably it was not a regular problem, but could potentially have caused a few issues. Addition of independent traction control could have mitigated this problem as would adding an extra TM ( will increase weight and also lower the effect...
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of the reaction torque).

The WDP4 has an erratic adhesive weight distribution due to Bo-1-1-Bo wheel configuration. Although it has 6 axles, only 4 are powered by motors. So, its adhesive weight is only 2/3 times its overall weight (119t). And besides, only the axles 1, 2, 5 and 6 are driven by motors, which means the adhesive weight is concentrated at the ends unlike a Bo-Bo type loco whose weight is distributed throughout and is more compact with a shorter wheel base which actually improves the loco's ability to negotiate curves. A gradient is a vertical curve basically. WDP4 is quite long (in terms of length) and has a longer wheel base (which practically suits a Co-Co type loco) for a loco with just 4 motors.
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you said, the front wheels tend to rise up due to reaction torque - this is observed particularly while accelerating on an incline. The slip is induced in the leading axles (1, then 2) first. WDP4's strength lies here - it can keep its wheel speed controlled even when the wheels slip due to creep control mechanism. So, it was (and still is) used widely in SWR and SCR.
Individual axle control could have solved it somewhat. But the WDP4 was originally intended to be a Bo-Bo type loco. It was converted to Bo-1-1-Bo because our tracks can't handle an axle load of around 30 tonnes. But this wheel configuration also had some limitations.
That's why a Bo-Bo or Co-Co wheel configuration is used normally.