Over 1 km (0.62 mi) of the line passes over water bodies in Bhakti Park and Wadala, both areas classified as CRZ II, where regulations limit construction activities.

The MMRDA proposed building metro car depots for the line on a 30 hectare plot at Owala village in Thane district and another in Vikhroli.

[22][23] In May 2019, MMRDA Metropolitan Commissioner R.A. Rajeev announced a modified plan to use Siddharth Colony as the interchange station with Line 2.

[24] The MMRDA considered plots in Wadala, Aarey Colony and Gaimukh Nagar in Thane to build a depot for the line.

[25] On 16 October 2023, the state urban development allotted a 174 hectare (435 acre) plot of land at Mogharpada in Thane to the MMRDA at no cost.

A joint venture between Reliance Infrastructure and Italian firm Astaldi S.p.A. was awarded three packages between Kasarwadavali and Wadala worth ₹1,584 crore (US$180 million).

[35] The contract for the remaining two packages, worth a combined ₹1,048 crore (US$120 million), was awarded to a joint venture between Tata Projects Limited and China Harbour Engineering Company.

[39] In September 2019, the MMRDA awarded a ₹342 crore (US$40 million) contract to J Kumar Infraprojects to build the viaduct and two stations for the Metro 4A extension from Kasarvadavli to Gaimukh.

[40][41] In August 2020, the Bombay High Court granted the MMRDA permission to carry out construction in a mangrove buffer zone.

[45][46] The Bombay High Court dismissed two petitions filed by Indo Nippon Chemical Co. Ltd. and Shree Yashwant Co-operative Housing Society Ltd. challenging the alignment for Line 4 in March 2023.

[47] In September 2023, the MMRDA awarded a ₹905 crore (US$100 million) contract to a joint venture between SEW Infrastructure and Vishwa Samudra Engineering to build the depot at Mogharpada.

It also awarded a ₹121 crore (US$14 million) to Apurvakriti Infrastructure Pvt Ltd. lay ballastless railway tracks connecting Mulund Fire Station with Gaimukh and the depot.

[18] The Maharashtra Government signed an EUR545 million (₹4,000 crore) loan agreement with German development bank KfW to fund construction of Metro 4 and 4A in November 2020.

[53] The MMRDA awarded a ₹1,854 crore (US$210 million) contract to Alstom to supply 234 metro coaches for Line 4 and 4A in March 2021.