A patient presented with pain in left eye associated with visual distrubance, also a history of blunt trauma to eye 4 month back, first investingation of choice is -
Patient is presenting late onset visual disturbance after trauma. Causes of delayed visual disturbance after trauma are retinal detachment, cataract and glaucoma.

"Late effects, including cataract, retinal detachment and glaucoma, all may result in visual loss. Therefore, continued follow-up care is important to preserve vision after an injury". — Ocular trauma

The symptoms of retinal detachment usually start as floaters in front of eye followed by visual disturbances. Condition is painless.
Cataract causes gradual vision disturbances, but is painless.
So, we are left with glaucoma only, which is one of most common cause of delayed vision disturbance after ocular trauma. Therefore, IOP measurement is the best answer here.
Intraocular tension
Ophthalmoscopy
Perimetry
Ultrasound