First of all note that Firefox 4 is no longer supported or considered secure, so you would do better to upgrade to Firefox 5 (Or temporarily downgrade to firefox 3.6 which is still supported - maybe considering a portable version or an additional custom instal)

However although I do not use Web Developer Toolbar myself I do note it appears to be compatible with versions of Firefox up to 6, so maybe all you need do is update the version of the add-on.

is there any plugin/extension available like Web developer toolbar for firefox 4?

I don't use the extension anymore, along with a serious problem in the past, it wants too many keyboard shortcuts which obviously conflict with others, and is a very large extension. Instead I use keyworded bookmarks for most such things and they work on any of my profiles without installing extensions. In fact the extension had its roots in a collection of bookmarklets.

The developer has it marked as good through 6.* and you are 4.0.1, and the current Firefox is 5.0

• https://addons.mozilla.org/firefox/addon/web-developer/

Firefox 4+ versions have a Web Developer feature built into Firefox. I haven't used it myself and don't know if will be suitable for your purposes or not.

It can be accessed from the orange Firefox button > Web Developer or from the keyboard using { Ctrl + Shft + K } which toggles it on and off. I can't seem to find any documentation about its features, I guess it's another Firefox feature where you have to fiddle with it to figure out how it works (pox on developers who don't document what they create).