Unless a soil test indicates otherwise, 4 of these spikes twice a year seems like a really excessive amount of fertilizer even for large rhododendrons. It would be much less expensive to use Hollytone or another granular fertilizer intended for rhododendrons. Since the spikes are working well for you, it's undesrtandable that you might be reluctant to change. A simple soil test may show that the money spent on spike type fertilizers in this quantity is largely wasted. It would also provide guidance about how much fertilizer - if any - is really needed.

Rhododendrons do not need much if any fertilizer. Nurseries force their plants by feeding frequently, but this often delays flowering. Once you use a fertilizer with chemical nitrogen, your plants will need more fertilizer than normal and have difficulty using iron, an essential nutrient. The problem is that chemical nitrogen is hard on mycorrhiza, a type of fungus that works closely with the roots to bring iron and other nutrients from the soil into the plant. It is best to use either no fertilizer or one with organic nitrogen like Hollytone or cotton meal. If you are having any chlorosis (yellowing of leaves between green veins) you may have to increase the acidity and/or use an iron supplement, especially a chelated iron (a form that is readily used by plants).

Rhododendrons have very shallow roots, so a good granular fertilizer with organic nitrogen spread over the surface inside the drip line is the best approach. Your plants should be mulched, so you can either just let the fertilizer work its way down through the mulch, or help it. But never cultivate the soil. The roots are shallow and easily damaged. Many time the roots will actually work their way up into the mulch.

Here is a link that might be useful: How to grow rhododendrons and azaleas

The best fertilizer for rhododendrons, in my opinion, is a top dressing of compost. I usually use aged wood chips.
Fertilizer spikes burn the feeder roots that come in contact with it and don't do much between the spikes. As said above, rhododendrons don't require much fertilizer, if at all. I never fert mine and I grow a lot of them. The texture of the root medium is much more important than fertilizer. It should be loose and damp.
Mike

Here is a link that might be useful: My garden