This is very safe. There is excellent infra and connectivity between at NDLS, IGI Terminal 3 and between them. You can deboard at NDLS between 4-4.30 AM, Go to IRCTC lounge on PF no. 16, refresh yourself, and right across the parking lot, there is Airport Express Metro Line which connects NDLS directly with T3. It takes hardly 20 minutes to travel. The first metro leaves at 4.45 AM (2nd at 5.30 AM), thereafter every 20 minutes. Fare is 60 or 80 per head.
Do not even think of taking Taxi / auto at that ungodly hour in Delhi (Ola / Uber is okay). They will rip your wallet apart.
I...
more... should say you have the best buffer, and you happen to be at the best railway infra (lounge and all), best public transport (Airport express metro) and best airport (T3) in India