A mic signal is very low, unless it's preamped. By definition it's not a +4dB signal. Since you are connecting your mic to the Mackie, it should be preamplified by the channel preamp, if connected to an XLR mic input.
I'm not sure the split cable is a good solution though!
Another aspect to consider is that when using XLRs the Eventide is expecting to "hear" +4dB line level signals; since the input headroom is quite large, you may want to boost the SEND to the 4000 or the input gain on the 4000 to "help" pushing the input signal.
all the best