I've been trading cryptocurrencies for a little while now, and recently I have been looking into stocks, futures and forex etc. During my research, I have run into a subject called settlement times. I have been told that settlement times for most stocks are t+3 / t+2, and settlement times for forex is t+2.

Lets say I open a cash stock trading account and I have $8000 in it. Is is it legal to  do a $4000 trade in one stock, sell that stock for a total of lets say $4100 after 1 hour, and then proceed to buy $6000 of another stock in the same day?

Also, how would this apply to forex and futures?