Is she interested or just busy?

So i found a friend from 4 years back, on tinder. but i decided so send her a text on messenger since we are friends on facebook. i started slow, saying hi and she replied almost instantly, asked normal questions like long time no see, how are you doing and what are you doing lately. she looked happy when answering me with emojis and stuff. i wanted to build up the convo to ask her out for coffee. she often replied pretty quickly, and i did almost the same, i couldn't answer too quickly since i was at work. but the last question i sent she never answered, she hasn't even seen it and its been 4 hours now, and i have seen her online many many times since. (i know 4 hrs isn't much but we had a conversation going) i'm so confused, and i haven't asked her for coffee yet either.
Is she interested or just busy?
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