Chances are that the precision afforded by your mobile device(s) that are outfitted with consumer-grade GPS won’t be any better than the positions afforded by the Inspire 1. So buying an app for whatever system, iOS or Android, isn’t going to overcome the limitations of this type of receiver-antenna built into your phone.
Depending on your mapping needs, you could even proceed without GCPs reliant solely upon the geotagging done by the DJI Inspire 1 while ever mindful that positions will be approximate; e.g., horizontal uncertainties likely within 3-5 meters and vertical uncertainties varying wildly. We’ve routinely seen ridiculous values up to hundreds of feet from the Inspire 1 between the geotagged altitude and the post-processed results with solidly surveyed GCPs. On other days, and quite unpredictably, differences will be more believable when expectations are tempered by the fact that Inspire 1 uses its barometric sensor to write z values while geotagging.
If you have GIS tools in your kit, access to professionally produced real ortho rectified imagery, good terrain data (LiDAR, DEMs, etc.), you can produce quasi-GCPs from those previously produced spatial products. The quality of that source data will largely impact the end result of your Pix4D project in a similar fashion as flying at different altitudes effects the Ground Sample Distance (GSD).
To achieve the best results, GCPs need to be surveyed. Typically, this will be done using GNSS RTK techniques which requires a rover unit that is receiving corrections from a base unit and its radio transmitter. Ideally, the base is on the project site, but corrections can also be received by many RTK rovers over the Internet using cellular communications and the base station CORS network frequently maintained by state departments of transportation. The local base station being the most robust (and expensive) choice. Alternatively, GCPs could be located using GNSS Static techniques which can produce very accurate positions after its data has been post processed. Static surveying will often mean longer occupation times too but won’t cost as much.
Before buying anything (it’s all quite expensive!) I suggest you determine the level of precision your projects require and then direct energies to learning about the tools needed in achieving those targeted specs. Your greatest dividends will be from the time you invest educating yourself before the purchase.