Hey Shraddha, The best and easiest way is through train ... take an overnight train from Delhi to Haridwar ... Catch an early morning bus to Auli or joshimath from Haridwar ... there are daily buses plying from Haridwar to Joshimath ... From joshimath ... auli is 15 Kms only... Auli is also known as skieeing town of India ... Places to see are :- Mana Village the last Village of india is nearby to Auli .... You can also visit Joshimath and witness lots of trekkers dre as it is the starting point of many treks .... Also Badrinath shrine is also near to Auli ... You can also visit Tapta kund and Panch shilas and take a bath in hot water spring dre ... All these places can be visited on a daily basis as it is 50 Kms from Auli ... So have a great journey ... Happy Travelling !!!
You have only 4 days so it will be quite hectic to take and then bus as roads are not that good. If budget is not a constraint take a cab so that you can take appropriate break.