How much ram can you access? I bet you need to drop your cores significantly for this step. Here’s what I do for a 16 core high mem node (512gb ram)
#make otus for each Order individually, for very large datasets (hundreds of samples) you may need to decrease the taxlevel to 5 or even 6. If you use 6 you will likely only get 3% OTUs because the within group differences aren't always 5%
cluster.split(fasta=current, count=current, taxonomy=current, splitmethod=classify, taxlevel=4, cutoff=0.15, processors=16, cluster=f)
#if your data is very diverse you may end up with .dist files that are too large for the number of processers (if you are using 4 processors your dist file needs to fit in RAM 4 times. For my system, I have to drop the processors in this step if my largest .dist is over 64GB)
cluster.split(file=current, processors=4)