Hello! I have just bought a Line 6 PODHD500 to hook up to my Fender Mustang IV to hopefully get a bigger variety of sounds. What I want to do is to somehow plug the PODHD500 into the Fender Mustang IV so that i can only use the effects, if that makes any sense. I like the sounds coming out of the amp and I would use the PODHD500 as a small pedalboard to click on and off phasers and flangers and an occasional wah pedal. Any advice with how to achieve this would be much appreciated thank you!
Doesn't the Mustang have all of those effects in it already?
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Using a multi-FX unit (or even any pedals, really) with a modelling amp isn't a good idea, it'll sound mediocre at best.
To get the best results, either use the Mustang by itself, or get a good non-modelling amp to hook up the POD.

Why didn't you get the footswitch for the Mustang instead of the POD? It'd have been a much better choice.
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That is a modeling amp so results may vary.

Try the Line in on the Fender (or I think they call it aux), you'll have to get a 1/4 to 1/8 coverter but you'll bypass the preamp, might like what you hear. You kinda bought two of the same thing, but by different company. You'll get better results getting rid of the Fender and adding a clean solid state amp, like the Tech21 Power Engine:
I used a POD HD500 into a Mustang III for a while. The POD totally slays the Mustang in terms of DSP quality, so you're better off using the POD for amps and effects, and the Mustang as a powered speaker.

I tried the loop return on the Mustang, which works fine, but I preferred to go in the front on a clean Mustang patch, (deluxe reverb with flat EQ and no effects) so I could still use the EQ knobs without affecting my POD patch.

I later got tired of the directional treble of the Mustang (a problem of all 12" speakers) and got a FRFR PA speaker (QSC K10) which was much better, both in terms of the frequency range and the spread of the sweet spot.