Concurrent of 4 ac of yours is ok,  but you have to aware of the situation if  by any time switch on two of the water heater you will risk tripping the main fuse/burned the main fused (if older type of fused).  Also make sure all the water heater and ac run independent circuit any sharing will risk burning the wires (worse your house)

The outside main fuse you can get it for free by calling TNB.

The outside fuse can buy outside about 10-15 bucks for 63amps.

Changing fused allow you to have fuse to take higher load,  but doesn't mean your original wiring can support it. Is better to confirm your wiring can support. From meter to house db box.

Outside is simple, db box do need some skills.

Its a bit expensive..

- He wanted to replace the old cutout fuse (next to the Tenaga KWH meter) and Neutral link just because its old. I would too ( Those are rm30 (60A HRC fuse & fuse holder, rm5 for the Neutral link)
  (15min job)

- Assuming he wanted to replace the rm30, 40A RCD (Maxguard) to match the load of the 60A DP Isolator (maxguard), (another 10min job)
Both components comply to the IEC regulation but thermal overload trip do not match each other..

Maxguard is a known electrical brand, its competitive and it has SIRIM/ST approval.. It has a presence in our local Malaysian market..

Those are just unscrew, component replacement and tighten back connections..

You can get another quotation if you want.. (A more reasonable rate would be around rm200)..

Electrical have rule to follow. Not what you want 100A this and that.

Max single phase is 60A.  So everything must be content in 63A. Fuse or isolator. More than that, advise you to get 3phase.

Your single room heater and aircon share the same point is not recommend. Even you change the MB to 40A, your wire spec exceed the actual current running. Heater 3.6kw and aircon 1.5HP 1.2kw = 4.8kw. Your 2.5mm wire is running hot.

As a precaution, separate into 2 point with 2pcs MB is the proper way.

I think you looking into burning your house.

Like i comment before, changing fuse is simple, can even put in 200Amp if there is one available, but can your wire handle the load?

One heater Max load is 15Amps, you switch on 15amps x 3 = 45amps,

1HP ~ 4Amps, 3.5hp =  14 Amps

You do the math

Option 1:
Ask wireman check the thickness of wires used, and use the correct recommended fused for the thickness wires you have.

Option 2:
Change the the wires that support 100Amps as you use 100Amps fuse.

What you earlier did was having a car that can run 100MPH, but the road was earlier tested on 40MPH. You either change the car to suite the road, or change the road to suite the car. Is your option.

You're right..

A HRC fuse protects the immediate wiring subcircuit..

Based on this logic..

100A single phase requires wire size to be 2x25mm2 copper cable or bigger for Live & Neutral .. means from 100A HRC fuse to TNB Kwh meter to 100A Distribution Board (DB)..

So this means your inside your DB must have a 100A Double pole (DP) to 100A RCD (to outgoing SP mcb Live busbar and to Neutral Bar) all still using 25mm2 wire ..

The only problem to this is the normal wire size that can fit inside a residential DB is only 10mm2.. which is only rated to 40A

If you know what your wireman is doing and he is indeed replacing the wire it is all good..

I am saying it is very hard and almost impossible to fit a 25mm2 pvc copper wire within a residential DB and I think your wireman is taking you for a fool..

Another thing is the combined (maximum demand) load which can trip a 40A mcb will not blow a 100A HRC fuse..

What usually happens if a fault happens is a high fault inrush current(usually from AC motors) will melt fuse the 40A mcb contacts making it unable to trip heat up the 10mm2 copper cable and start to burn ..

This fault current will not trip a 100A HRC fuse (which is a slow thermal response high rupturing capacity type and not a magnetic type tripping device)..

A fuse blows open and mcb (miniature circuit breaker) magnetically trips (and a bimetallic thermal trip but the primary trip function is magnetic)..

The decision is yours to make but if you do it wrong  it is a safety issue ..

Edit * If you refer to the Suruhanjaya Tenaga tables for current carrying capacity the minimum cable size for 100A (enclosed in conduit) is 35mm2..

You must tighten the cables to make good contact if you know you have high loads or the connection points will heat up, start to arc and loosen in time..