Going natural gas is a no brainer. Do it. The question is 1 system vs 4. You can get away with 1, but you'll likely save in the long term with 4 separate units and pushing the costs to the tenants. I would only consider 1 system if you don't have the cash for the increased cost and you have higher end tenants that aren't going to abuse it. For the individual systems, these are on-demand, right? Hope you're not installing hot water tanks for everyone.
I would also consider if you have any common space heating registers - hopefully all common space heating will be tied into one of the 4 units. You may consider having that 1 system paid by you, so in any case of no tenancy or payment issues by the tenant, your building doesn't have freezing common areas. Alternatively, you're probably expected to disclose/rebate the tenant for paying for common area heating.
Another side benefit is in the case of a break down, especially during the winter, your entire building isn't going without heat.