Reread your question a third time. So you want to cheat your 1000 W ballast into a 400 W one. For starters, that aitotransformer “input” side of the ballast is partially or entirely in order to overcome the working or forward voltage of the bulb, which at 1000 W may be substantial. Please compare the datasheets for both 400 and 1000 W MH bulbs and find working voltage for both. You probably do need to calculate or simulate it from there since you have several things going on: overcome Vf, too low inductance for 400 W bulb and “cheat” with input voltage.

@winny Yes, exactly! I'm trying to see if it is okay to do so. Yes I've to admit that that the 240V wiring on 120V causes the light to reach to its full output only after like 2-3 mins approx. When using the 120V wiring it reaches the full brightness in like 3-10 seconds. That kinda scared me like will the bulb explode or something. I've also added the ballast comparison between a 400W & 1000W ballast.

You will probably need something in-between. Can you cheat and wind a few more turns on the choke side, or add more inductance in series?

I think the later option of adding an inductance is possible. Re-winding the core is a hassle. I would just get a 1000W bulb which is easier. Unfortunately, most shops now sell only MH lamps and that too only the lamp not the ballast as they say its banned and being phased out due to huge power draw.

Get yourself a current clamp meter and measure the lamp current when warm and compare with the datasheet. I would expect 3.5-4 A. Too much? Add inductance. Too little, remove inductance. If you are not afraid to hack together something, you can parallel (identical) ballasts to increase the current capability and series connect them to get more inductance.

@winny So, did a small current test today. At the moment I don't have a clamp meter with me. I've only a series type AC ammeter as shown in the above picture. It's reading is like 4.3A after the initial strike were the current was like 6A for a second .I'll try to borrow a clamp meter from work tomorrow and see if the reading is the same. That current seems to be what is expected from a 400W ballast it seems from the above spec chart comparison. So, should I just leave it as it is?

Great! Clamp meter was only to save your DMM from any high voltage ignitor, although in current reading mode it should not be a problem. Anyway, if it survived then no need for clamp meter. 4.3 A sounds a little bit high but if you can live with the colors being slightly off spec and a bit shorter lifetime, I would not worry about it.

As sown in the figure its a 10A ammeter. So, I was hoping that it would survive. But just to confirm I'll take readings with a clamp meter tomorrow and post the results. Also the bulb has a label says

it works with ballast & ignitor. But this is an old ballast and doesn't have an ignitor capacitor.

Sorry fro this noob question. But doesn't MH lamps need ignitor only below 250W?