With a written contract, the statute of limitations is five years. Thus, as long as the landlord acts to collect rent owed under the lease within the five year period, the action is not time-barred.

If the landlord was not assessing taxes against the tenants until four years ago, and never actually collected taxes, and that is true of all tenants, then the tenants may be able to argue that the tax provision was either waived or was included by mistake; although if it's clear in the lease it will be difficult to prove mistake. If all tenants are aggrieved, then it may be possible to work out a collective resolution.

Your lawyer's position is understandable, because the law is not favorable to a business that claims that it did not understand or notice the plain language terms of a contract. It's your obligation to read a lease before you sign it and, unlike a residential tenant, as a commercial tenant you are deemed to be legally sophisticated when you engage in contract negotiations.