Im not going to speak for Ellenson but I can provide some context for Brown and you can make up your own opinion.
Yes Brown can play small ball 4. He is very similar in size to Crowder. Ok now to the context. So to start the Cal season, Brown was asked to play the 4. They had Wallace starting at point and 2 juniors Matthews and Bird starting next to Brown and Rabb. There were instant problems. Brown and Rabb were young. Brown is a power player. Rabb wasnt used in the high post but the low post exclusively. With that being said that wasnt the problem. The problem was that Wallace is not a good shooter and Bird and Matthews the shooters werent hitting shots so the court became very small.
Phase 2. Brown slides to the 3 and they start Rooks at the 5 who has no range. So you have Rabb and Rooks on the block. Brown who still was really inconsistent with his jumper at the 3. Matthews at the 2 and Wallace at the 1. Oh and btw, Wallace isnt a natural lead guard so he drove to the lane constantly.
Phase 3. Wallace gets hurt and Brown plays sudo PG and a shooter and distributor Singer starts at PG. At this point Matthews and Bird start to hit shots. Singer passes the ball and Brown plays more of a 3 until they need more space and take one of the bigs off the floor and have Rabb solo on the block.
Phase 4. Wallace comes back but Brown is alot better at this point but when they need baskets they take a big off the floor and Brown plays the 4.
So yes he does play the 4 against non tradition 4s. I will leave you to draw your own conclusions and why he plays there and what could be done differently. If you want me to share how I see this playing out while watching the games just ask. Dont want to cloud peoples opinion 1st thats all.
Interesting article about playing Brown as a PF and Ellenson as a C...
http://basketball.realgm.com/analysis/240831/Jaylen-Brown-Henry-Ellenson-And-The-Benefits-Perils-Of-Sliding-Positions
Could help them standout even more.