Employer maintained a prior plan and now wants to setup a new plan
Can service prior to effective date of new plan be excluded for vesting purposes?
IRC 411(a)(4) seems to say "no" but I vaguely remember there being exception to this, perhaps a 5-year rule?

Vesting must be credited for all years where another plan was maintained unless the old plan is not what is referred to as a "predecessorr" plan. The old plan is not a predecessor plan with respect to the new plan if the old plan was terminated more than five years before the new plan was set up. See 1.411(a)-5(B)(3)(v)(B).