[milo] Reduce parallel requests from 8 to 4
[milo] Reduce parallel requests from 8 to 4

Bug: 806700
Change-Id: Ie003fc77894d2e4217a913dc6ae2e3b093064d3a
Reviewed-on: https://chromium-review.googlesource.com/891631
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Commit-Queue: Ryan Tseng <2190fe9a39b4920ec7bd59c848ec5cccd163056f@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go