How to compliantly bond galvanized water pipes if the water heater isn't an option
Accessible means &quot;no demolition needed&quot;
The definition of &quot;accessible&quot; as applied to wiring methods (vs. equipment) is fairly simple:

Accessible (as applied to wiring methods). Capable of being
removed or exposed without damaging the building structure
or finish or not permanently closed in by the structure or finish
of the building.

As a result, having an access hatch, cover plate, or such that needs to be opened to access the bonding connection is not an issue.
Bonding and grounding are two different things
Your situation, with a galvanized service line but a mix of metal and plastic interior piping, does put an emphasis on the distinction between grounding to a water service and bonding your water piping to ground.  In houses with all-metal plumbing, all-plastic plumbing on a metal service, or all-metal plumbing on a plastic service, this is handled through a single connection to the water system. This leads people to ignore the distinction, but since you have a mix of plastic and metal plumbing on a metal service, you need to pay attention to what you're doing insofar that a bond at one point won't be sufficient.
You'll need something fatter than 4AWG, too
The other problem is since you have a 400A service landing on service equipment on the exterior of the house (vs. a &quot;maypole&quot; service where the service equipment's on a pole with the meter), you need to size the bonding and grounding system accordingly.  While a 4AWG copper conductor is adequate for grounding to ground rods or an Ufer electrode due to the NEC 250.66(A) and (B) exceptions to normal grounding electrode conductor sizing, water service line electrodes have no such exception, so its grounding electrode conductor needs to be the full size required by your service, or 1/0AWG for an overhead 400A service (600kcmil total area) as per table 250.66 in the NEC, and also needs to connect to the plumbing within 5' (not 10') of where the service line enters the building as per NEC 250.68(C) point 1:

(C) Grounding Electrode Conductor Connections. Grounding electrode conductors and bonding jumpers shall be permitted to be connected at the following locations and used to
extend the connection to an electrode(s):


(1) Interior metal water piping that is electrically continuous
with a metal underground water pipe electrode and is
located not more than 1.52 m (5 ft) from the point of
entrance to the building shall be permitted to extend the
connection to an electrode(s). Interior metal water
piping located more than 1.52 m (5 ft) from the point of
entrance to the building shall not be used as a conductor to interconnect electrodes of the grounding electrode
system.

However, since you have metal pipework that's not electrically continuous with the service line, you also need to bond that piping to the grounding electrode system or service equipment as per NEC 250.104(A)(1).  Fortunately, the Table 250.104(C) requirements for bonding jumper sizing also require 1/0AWG wire for your application, so you don't need yet another size of wire, at least.  Furthermore, unlike grounding electrode conductors, these water system bonding jumpers do not need to be run without splice or joint, so that gives you much more flexibility as to how you arrange them.  The caveat here, though, is you can't just bond the cold water pipe and call it done; that &quot;island&quot; of metal hot water piping must be bonded as well, even if you have to make the bonding connection under a sink or such.