stats and artifacts you use will make a huge difference towards ur champions overall stats, you said for example you are using fully maxed 4 star gear (try to avoid maxing out 4 star items, you will heavily heavily regret this in the later game) lets take a look at example a 4 star chest plate with example hp% as the main stat, even fully maxing out this 4 star item would yield 40% total bonus hp, now if you get the same item and its 6 star, that main hp% stat can go to 60%, as you can see this is a 20% difference on hp to ur champion in just 1 single item, now also depending on the "rarity" of ur item aka if its rare (blue) epic (purple) legendary (gold) will depend how many EXTRA substats rolls you get, start applying this logic to all ur items and u will soon notice ur stats can improve dramatically, but dont be frustrated this games designed to be grind and you wont get these items over night. also as others have said you can use "off set items" what it means is example if ur character ur using is example using lifesteal set , put them in the best life steal items you can (4 for set) and then the last 2 items just find the absolute best stats that would benfit ur champion the most and add those (it doesnt matter if the last 2 are a set or not just aim for what gives you the best stats for that champion)
and again cant stress this enough, try not to max out a 4 star item unless you absolutely have to, save ur silver for 5 star items at least, becasue you will at least be using good 5 star items a good majority into the game, 4 star items you will be looking to throw into the bin with all the silver you invested into it after a few months.