4 % is the maximum art. 102.3 TRLRHL allows a council. Many charge less: check the rate in your own fiscal ordinance.
The reliefs do not add up: each letter of art. 103.2 applies «to the tax resulting from applying, where relevant, the reliefs referred to in the preceding paragraphs».
Whether two reliefs can be combined is decided by your ordinance, not by the statute: the closing paragraph of art. 103.2 hands it that job expressly.
The tax accrues when the works start «even where the corresponding licence has not been obtained» (art. 102.4). Starting without paperwork does not avoid it.
Refine: second relief and licence fee deduction
Art. 103.2 chains the reliefs in the order of their letters, and its closing paragraph leaves it to the ordinance to decide whether they can be combined at all. Art. 103.3 allows the licence fee to be deducted, but only where the ordinance provides for it.
The rate and the reliefs are set by each council in its own fiscal ordinance. This calculator defaults to the statutory 4 % ceiling because it is the only figure the law gives; yours may be lower.