From the start, I haven't pruned my peach tree like I should have. It's about 4yrs old now and overgrown. Is it too late to take control?
It isn't too late. Since you won't be able to keep any first year branches, you will not be able to get much fruit next year. Following that, with regular pruning, you should start to see good crop production.
https://www.gardeningknowhow.com/edible/fruits/peach/peach-tree-pruning.htm
Prolific--fruit set on in clusters like grapes. Largest ripe ones almost 3" tall. Orchard is in Klickitat County, WA. Sorry. No photos taken.
Without photos, this will be very difficult. In this case, I would suggest taking samples or photos to your local extension service. They may be able to help:
https://www.gardeningknowhow.com/extension-search
Here is a collection of articles on peaches that you may find useful:
Last year my reliance peach tree after giving bountiful fruits for several(at least3years)years, did bloom and though flowers became fruits but they did not grow in size and most of them withered away What can be done to treat it. Local garden centre suggested that it is because of the lawn fertilizer. I also have brown rot and leaf curl problem and I sprayed the tree with dormant oil and sulphur twice this year.
If your tree is in the lawn, it could be getting too much nitrogen if you fertilize the lawn. Use a balanced fertilizer such as 10-10-10 for peach trees. These articles should help:
https://www.gardeningknowhow.com/edible/fruits/peach/peach-leaf-curl.htm
https://www.gardeningknowhow.com/edible/fruits/peach/peach-brown-rot-control.htm