Osteoarthritis (OA): Given the patient's age and the location of the pain (4th MCP joint), OA is a common condition that could explain the symptoms, especially pain with flexion and extension without swelling or history of trauma.

Gout: Although there's no swelling, gout can present with joint pain without overt inflammation, especially in older adults. The absence of a clear history of gouty attacks or urate crystals does not rule out this diagnosis.
Psoriatic Arthritis (PsA): Though less common, PsA could be considered, especially if there are other signs of psoriasis or a history suggestive of inflammatory arthritis. However, the lack of swelling and specific mention of pain with movement makes this less likely.
Abiraterone-induced joint pain: Abiraterone, a medication for prostate cancer, can cause joint pain as a side effect. Given the patient is on this medication, it's a plausible cause for his symptoms.

Septic Arthritis: Although the patient does not present with the classic signs of septic arthritis (e.g., significant swelling, redness, warmth), this condition is a medical emergency and can present atypically in older adults or those with compromised immune systems. The absence of swelling does not rule out this diagnosis.
Metastatic Disease to Bone: Given the patient's history of prostate cancer, it's crucial to consider the possibility of bone metastasis, which could cause localized pain without swelling.
Osteonecrosis: This condition, which can be related to steroid use (though abiraterone is not a traditional steroid, its impact on steroidogenesis could potentially contribute), presents with joint pain and could be a consideration, especially if the pain is persistent and localized.

Hemarthrosis: Though rare and typically associated with trauma or bleeding disorders, hemarthrosis could be considered if there's an underlying coagulopathy or the use of anticoagulants (not mentioned in the scenario).
Pseudogout (Calcium Pyrophosphate Deposition Disease, CPPD): This condition can cause acute or chronic joint pain and might be considered, especially if there are risk factors such as advanced age, though it's less common than other diagnoses listed.
Sarcoid Arthritis: A rare condition that could cause joint pain, though it would typically be accompanied by other systemic symptoms or signs of sarcoidosis.