If the hydrangea has been in the same soil for four years then you need to repot or, better yet, plant in your garden. In general most plants do better in the ground.

Hydrangea petiolaris? If it is a climbing hydrangea it is a useful low-maintenance climber for a shady or north wall, and will also thrive in sunnier sites in moist soils but the flowers do not last as long. These plants are slow to get going, and often make little new growth in the first few years, but is well worth the wait. Certainly needs repotting at the very least.

Mary, you sent me a PM regarding your hydrangeas, but I'm going to respond in public for other's information as well.
As Moon Grower has already suggested, its likely that, after 4 years, the pots are now too small for the plants, so the first thing to do is turn them out of their pots and inspect the rootballs, also checking for unwelcome beasties living inside them. If they are root bound, you will either need larger pots, or to plant them in the ground.
You also say you feed them regularly with Seasol - you don't say which version of Seasol you're using (plus or Extra) but either way, I'm unable to find a chemical composition for this product online - I'm interested in the NPK, which should be on the label somewhere, and also precisely what 'regularly' means, and whether you're using it as a foliar spray or applying to the compost.
The other thing you mention is that they are in full sun, but shaded when very hot - not sure how you're managing that, but are the other hydrangeas you have in the ground also in full sun, or part shade? As you've said they're not mophead hydrangreas, nor that they climb, I'm assuming you've got lacecap varieties, or H. paniculata (cone shaped flowers).
I'd also ask whether you've inspected the stems and leaves for signs of infestation other than the occasional leaf spot which you mentioned.