A single equal sign is used to assign values to variables and a double equal sign is used for comparison.
Even though your code works you can completely simplify it by taking out the first half of your condition.
This example would accomplish the same task only if you wish to simplify and clean up some code.
client.on("cheer", function (channel, userstate, bt, message) {
console.log(chalk.blue(" Cheers Accepted " + userstate.bits));
if(userstate.bits <= 999){
bt = "t1"
}else if(userstate.bits <= 4999){
bt = "t2"
}else if(userstate.bits <= 9999){
bt = "t3"
}else{
bt = "t4"
}
hue.hueLamp("bit_" + bt);
});