Apparently, the board of directors of Sprint are willing to let T-mobile invest in Clearwire to expand their network further, according to Wall Street Journal. Since Sprint owns 54% of Clearwire, it's up to them to fund Clearwire's 4G network. However, I don't think Sprint can afford a few more billions investment considering they are one of the smallest networks, offer some of the best plans, and they're already ahead of other carriers when it comes to investing in a 4G network.
This is why Sprint could let T-mobile give Clearwire the rest of the money they need to cover most of the cities in USA. Then both Sprint and T-mobile could share Clearwire's 4G network, and they will be better off than investing by themselves in their own networks. It's hard to compete with their much bigger competitors Verizon and AT&T.
So far, T-mobile has only been interested in HSPA+, which is a little strange, because it's obvious every carrier needs to move to 4G if it doesn't want to lose all its customers to those that do before them. So either T-mobile is waiting to ally with Sprint, or they simply decided not to invest in 4G for now.