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  Regarding the first point, your idea to set up two new servers is a fine one though a bit risky - especially if there are any problems during the switchover. If everything goes smoothly the day of the changeover, you're good - if not, there could be problems.
  Assuming you'll either have the new ISP up ahead of time or can keep the old ISP a bit after the transition (a day of overlap should be plenty, I would not replicate ns3/ns4 (the new servers) with ns1/ns2 (the old ones). Instead, I'd load only the DNS information needed to resolve hosts once assigned to the new IPs on ns3/ns4. Essentially, you'll still have 2 pairs of server and only one pair online at any given time. That way, as soon as you bring the new IPs online, the new DNS servers will be there and waiting with only the correct information (no stale records from old IPs).
  Be sure you register all 4 DNS servers with your registrar well before the swap so that all 4 are offered as authoritative even though only 2 will be available at any time.
  As for reducing the replication time once you make the change, a few days (or more) before you are ready to make the transfer, reduce the TTLs on all records (including the SOA) on the old servers to less than 30 minutes. Doing it well in advance will make certain that the new TTL will have propagated to any servers on the internet that may have cached records and those servers will know to expire the cache quickly and look for new info. That way when you do make the swap, the new addresses will be picked up quickly.
  This will increase DNS traffic quite a bit. If that will be a problem, set up an additional server with a third party (even if only temporarily) and make it the primary for your domain with your registrar.
  Hope this helps!
  Where I see a problem is that you host your DNS in-house, which means, when switching ISP, your DNS will be in dark, too, so your parent DNS at your registrar will fail lookup, and DNS changes will not even begin to propaginate.
  To workaround, as Mdcsea already suggested, it is a MUST TO setup external DNS, not only for the switching period, but also for later use. I suggest you to use free DNS service at, where you may setup Secondary DNS for your main domain, which hosts your DNS.
  For example, if you use your own DNS names
  ns1.yourMainDomain.com
  ns2.yourMainDomain.com
  setup Primary DNS for yourMainDomain.com in. I say *primary* because if it is Secondary, you won't have a chance to edit zone records to point to new IP, so temporarily zone *must* be Primary.
  After you make a switch, you may convert the zone to secondary with a simple!
  1.) Add secondary MX records for your domain with higher number (higher number=lower priority MX)
  1b.) If you want to speed up transition process, lower TTL values, also SOA Time To Live, in your old DNS zone settings. Lower TTL down from default 24 or 48 hours to an hour or two, or even less, but an hour is enough.
  2.) Setup your new server and make sure all is working fine, all websites, all databases, all mails
  3.) Setup your new DNS, and, if it is on YOUR domain AND will have different name as your old DNS servers, don't forget to register those new DNS server name, for example, ns3.yourdomain.com, at your domain registrar and tell them it is for DNS server! Registering a sub-domain, for example, ns3.yourdomain.com as DNS server, needs some extra steps at your registrar, yhich will register this name in DNS namespace.
  4.) Test your new server from your home computer, temporarily assigning your new DNS server's IP as your only DNS server for your home computer. Doing so, you will emulate as your new server is already in production, but only you will see it.
  5.) When everything works fine, you are ready to either:
  - change NAMEs of your dns servers at your registrar, if your old DNS was, for example, ns1.yourdomain.com, and your new DNS is ns3.yourdomain.com
  - or change IPs of your DNS servers, if your old and new DNS servers will have the same name, only IP will be different
  6.) After TTL time your transfer will be visible to whole internet and you are done. Hopefully with no problems.
  When you are ready to make your transition, simply shut down the servers that contain the old information. Remember though, if you don't set the TTLs down to a low number shortly before the transition you will experience a latency as cached DNS records expire on servers across the internet.