My (32 F) estranged brother (35 M) responded to a text for the first time in 4 years. It's the only chance I've gotten to reconnect. What do I do??

(Please be gentle, I'm genuinely panicking over what feels like my only chance to have any sort of relationship with my sibling.)
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For background: My brother cut me and our mother out of his life four years ago. He got married and his wife didn't like the two of us very much, and since his relationship with our mother had already been strained, her dislike was something of a last straw for him. He and I were very close growing up, but since his wife wasn't ok with him talking to either of us, and because I was supporting our mother financially, he cut me out alongside her.
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Before he cut us out entirely, I'd demanded to know why I was being cut out when his issues were with our mother, not me, but he never had a response for me. It always came back to "Well Mom did [x]." "But I'm not Mom." "Yeah, but Mom did [x]." My association with her was enough for him, but I wasn't going to turn my back on her to keep him, especially after we lost our dad and when his wife already had a huge hate-on for me.
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When I realized I'd lost him (he'd been responding less and less, and finally delivered a long text stating his grievances and that he wanted nothing to do with us anymore), I said some things I regret. I've always been the peacekeeper of the family, but I was extremely hurt, wanted to hurt him back, and felt at the time that I had nothing to lose. He gave me a brief response and has since refused to acknowledge my existence. The first year of our estrangement, I occasionally reached out a few times, small texts about something I was watching or the simple statement "I miss you". He never responded, and I didn't want him to block my number, so I stopped reaching out. I never apologized for the things I said in anger.
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Since then, I've been pretending he didn't exist, as much as that's possible when you've lost someone who partially defines who you are as a person. It hurts, but I've been getting by. A few days ago, I was at a party and someone sent me a picture of what I would look like if I were a man, and it was the spitting image of my brother. So, expecting him to ignore me as he's done for the last several years, I sent the image to him with the caption that it made me laugh and that I hoped he was doing well.
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He actually responded, with a short but positive message. And I'm now sinking into a panic spiral. Do I take the opportunity to apologize for the things I said? Do I pretend nothing happened and try to continue a normal conversation? I ask this knowing full well that his response was likely not an invitation to continue talking, and that anything I send to him will probably get no response, but this is the first ray of hope I've had in years with a sibling I was once very close with. Has anyone else had a surprise, temporary reconnection to an estranged family member whom they actually wanted in their lives? What do I do? How do I not make matters worse? For all my hurt feelings, I know that none of that matters if I want him back--he's perfectly comfortable keeping me out of his life, and I still stand a good chance of never seeing him again.
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tl;dr: My brother stopped speaking to me years ago and I stopped reaching out in turn. He shocked me by actually responding to a text I sent him. What do I do to at least try to make this a genuine chance to reconnect?


Source.