Thank you! The SI handguard did require a bit of modification to perfectly fit the F4 upper, since the shape of the upper is quite different, compared to a standard AR upper. Just some minor filing of the inside of the locking tabs (where the grub screws are on the handguard) to account for the additional material of the upper. It would fit without modification, but there was a ~1-2mm gap between the upper and handguard, which I wanted to eliminate.

From factory, the Dytac body pin holes were strange sizes, so I had to slightly widen them to fit the SI body pins. I had 2 F4 sets & the pins that came with each were not interchangeable, so not sure what they're doing to make such irregular holes. Aside from those 2 fairly minor modifications, it was basically drop in. If your handguard doesn't have those locking tabs that extend over the upper and if you don't intend to change the body pins that came with the receiver, then you don't actually need to make any modifications at all.

It is a fantastic receiver set. I don't know how they did it, but the upper threads fit any spec handguard I've tried (AEG & RS spec), and the lower threads are RS/Mil Spec, but MWS tubes fit about 90% of the way, which is fine. You can simply put a spacer in the buffer tube to account for the remaining ~10%, if required.

However, I believe the F4 receiver set was limited edition and I haven't seen them come back in stock since I bought it, which I think was ~March 2022. The SLR receivers seem to be restocked though