There is a distance of 600 km between my house and my workplace. I stay at my workplace for about 20 continuous days each month, what is the ruling on my swalah at my workplace in terms of shortening or completing if I pray alone or as an Imam bearing in mind that 4 years has passed while I was shortening, what is the ruling of my swalah during this period? هل لي رخصة المسافر؟

Praise be to Allah, and may Allah’s peace and blessings be upon the Messenger of Allah, his family and his companions.
Wa alaikum assalam wa rahmatullahi wa barakatuh.
As to what follows:
With regards to answering your question we say, success is by Allah Almighty.
There is a difference of opinion among scholars regarding this issue. The majority of scholars specifies the period of stay whereby shortening is permitted for a traveller, with differences among them; Three, four, ten or fifteen days. All these estimates do not have any proof supporting them hence the preponderate view is that as long as a traveller has not adopted a place as his homeland or domicile he should shorten even if he is staying for a longer period.
From the most prominent people who supported this view is Shaykh al Islam Ahmad Ibn Taymiyah (may Allah have mercy on him). Shaykh Muhammad Ibn Uthaymin wrote a letter confirming this view and inferring from it so I would advise you to go and look at it.
And Allah knows best.

Your brother,
Dr. Khalid al-Muslih
10 / 02 / 1425 AH