I can't imagine WebM impacting 4chan anytime soon, it will probably reduce 4chan's load when(if) WebM takes off but I highly doubt more than a small fraction will choose WebM over a .gif in the immediate future.
WebM has caused longer animated content to be posted that wouldn't have been so possible as gifs (the limit is at 2 minutes iirc), so there will still be people posting large files.
That is technically correct but does not covey the scale at which 4chan operates. According to http://www.4chan.org/advertise,
Makes me wonder if WebM will increase or reduce 4chan's total traffic (when measured in bytes, not clicks).