It may be a sign of either. Did he act like this when he was bonded? Rabbits often act like that if they are lonely (my only experience with rabbits exhibiting that behavior when they are not lonely is unspayed does having false pregnancies). 4 isn’t too late to bond. He will live another 6 or more years if taken good care of. I just bonded my 4 year old to a 2 year old, and it worked really well. Finding a bun to be a friend for an adult rabbit can be harder than finding a friend for a baby. Pay very close attention to personality types (you need 1 dominant and 1 submissive; bunnies need a pecking order), and make sure the bunny you get was not taken from a preexisting bond. Bunnies who are already bonded are very resistant to bonding with a different rabbit if they think their buddy is still around.

Hi!
It’s great that you’re considering all options before getting another bun.

Just because he had a bad experience with another bun, it definitely doesn’t mean he’s unbondable! Unfortunately it’s quite common for rabbits that have had a bad fight to hold grudges and not bond. So let’s forget about that past experience for now!

Does he prefer to be alone? Well, it is possible, but it’s quite unlikely. The vast majority of rabbits are happier bonded. That’s not to say single rabbits can’t be happy, and some rabbits do absolutely refuse to bond, but most rabbits are happy to have a friend of their own kind. I got multiple rabbits because even if I spent all my free time with my buns (not practical if you want friends/family!) after sleeping and work, that would still only be about 6 hours of interaction. For me, my bunny being alone for 18 hours didn’t seem fair. Of course some people have different situations and are able to spend all day with their buns, so that’s slightly different.

I would suggest that the best next steps for you are to look for a rescue and see if they offer speed dating for buns! Then you can get an idea of how he reacts to other bunnies in a neutral space before bringing one home to potentially bond. You could also check if they’ll take the other bunny back if you’re unable to bond them, or even if they bond for you. You can also think about whether you have the capacity to keep to separate bunnies permanently (I assume not, since you rehomed Winnie). This is why it’s probably good to go down the rescue route.

The other thing i’d suggest is reading the bonding section in BUNNY INFO, so you have an idea of the steps. You can also read some journals in this section of the forum so you can see what you’re committing to!