The RIR and Light Sussex also keep digging holes to squat and roll around in while making weird clucking noises like they're growling or something... does that mean they're likely to be hens?
Sounds like they're just enjoying their dust baths. Could you post a full-body shot of your speckled sussex? By 4 months a boy should have pretty noticeable hackle and saddle feathers. Its combs looks boyish, but I'm not noticing male feathering from that angle.
I'll post a pic of the Speckled Sussex tomorrow, but I'm pretty sure it's a roo because it's had a huge noticeable red comb since about 4-5 weeks of age~