Shimano 7 speed with click shift not enough range. Can only get 4 of 7 gears
The problem is for sure in your shifter. Make sure both levers can move freely at any gear (i.e. are not hitting against brake lever mount or other accessories).

Also make sure that your shifter's inner mechanism looks fine, parts move freely. If not - consider lubrication.

Disconnect cable from the derailleur, take shifter off the handlebar, shift to the lowest gear and try shift 7 times up. If you can't so the only option is to replace it to proper 7 gear shifter.