The cumberbunds tend to be a popular place to put IFAKs and TQs. Since your carrier has elastic cumberbunds, you can't do that.
As others have said, there are a wide variety of "danglers" (things that velcro below the plate pocket) that you can use to place IFAKs and/or TQs.
Wherever you put your TQ or multiple TQs, just make sure you're able to reach your one TQ or one of your TQs with either hand, since obviously either arm might be the limb you're needing to apply the TQ to.
As others have also said, consider scaling down from 4 mags. 3 mags or maybe even 2 would realistically be plenty for most realistic needs and would leave room on the front of the carrier to place an IFAK or a TQ.
As for reinserting mags, look at Esstac Kywi pouches. They have kydex wedges inside them that grip down on the mags similar to how kydex holsters grip down on pistols, enough grip to keep them secure while still being able to easily draw and reinsert them.